Patent Publication Number: US-8983759-B2

Title: System and method for communicating in a vehicle consist

Description:
FIELD 
     Embodiments of the inventive subject matter described herein relate to communications between vehicles in a vehicle consist. 
     BACKGROUND 
     Some known vehicle consists include several powered vehicles that generate tractive effort for propelling the vehicle consists along a route. For example, trains may have several locomotives coupled with each other that propel the train along a track. The locomotives may communicate with each other in order to coordinate the tractive efforts and/or braking efforts provided by the locomotives. As one example, locomotives may be provided in a distributed power (DP) arrangement with one locomotive designated as a lead locomotive and other locomotives designated as remote locomotives. The lead locomotive may direct the tractive and braking efforts provided by the remote locomotives during a trip of the consist. 
     Some known consists use wireless communication between the locomotives for coordinating the tractive and/or braking efforts. For example, a lead locomotive can issue commands to the remote locomotives. The remote locomotives receive the commands and implement the tractive efforts and/or braking efforts directed by the commands. In order to ensure that the remote locomotives receive the commands, the lead locomotive may periodically re-transmit the commands until all of the remote locomotives confirm receipt of the commands by communicating a confirmation message to the lead locomotive. 
     Due to interference with wireless communications caused by other wireless devices, significant distance between locomotives, travel through tunnels or urban areas, and the like, some confirmation messages may not be received by the lead locomotive. As a result, the lead locomotive may continue to re-send the commands to the remote locomotives on a periodic basis, even if the remote locomotives have received the commands. Some lead locomotives declare a communication error or loss of communication state when all of the remote locomotives do not reply to command messages after a designated number of re-transmissions of the command messages. Such a communication error state alerts the operator of the loss of communication with the remote locomotives and may cause the operator undue concern about the operating state of the remote locomotives and the operator to slow or stop movement until the error can be examined and/or repaired. If the cause of the error state is that the remote locomotives are receiving the command messages but the lead locomotive is not receiving all of the confirmation messages from the remote locomotives, such an communication error state may be unnecessary and consume considerable time during the scheduled travels of the consist. 
     BRIEF DESCRIPTION 
     In one embodiment, a method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in the vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving the command message at the remote vehicles and transmitting reply messages to the lead vehicle from the remote vehicles. The reply messages include statuses of the remote vehicles that transmitted the reply messages. The method further includes receiving the status of a first remote vehicle of the two or more remote vehicles at the lead vehicle and receiving the status of a second remote vehicle of the two or more remote vehicles at the first remote vehicle. The status of the second remote vehicle is not received at the lead vehicle. The method also includes storing the status of the second remote vehicle at the first remote vehicle and transmitting a retry message from the lead vehicle to the remote vehicles. The retry message includes an identity of the second remote vehicle. The method further includes receiving the retry message at the first remote vehicle and transmitting a repeat message from the first remote vehicle to the lead vehicle in response to receiving the retry message. The repeat message includes the status of the second remote vehicle. 
     In another embodiment, another method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving a first reply message from a first remote vehicle of the remote vehicles in the vehicle consist. The first reply message is transmitted by the first remote vehicle responsive to the command message and including a status of the first remote vehicle. A second reply message is not received from a second remote vehicle of the remote vehicles in the vehicle consist responsive to the command message. The method further includes transmitting a retry message from the lead vehicle to the remote vehicles. The retry message includes an identity of the second remote vehicle. The method also includes receiving a first repeat message from the first remote vehicle in response to the retry message. The first repeat message includes the status of the second remote vehicle. 
     In another embodiment, another method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving a first reply message from a first remote vehicle of the two or more remote vehicles in the vehicle consist responsive to the command message. The first reply message includes a status of the first remote vehicle. If a second reply message is not received from a second remote vehicle of the remote vehicles responsive to the command message, then the method also includes transmitting a retry message from the lead vehicle to the remote vehicles (where the retry message includes an identity of the second remote vehicle) and receiving a first repeat message from the first remote vehicle in response to the retry message (where the first repeat message includes the status of the second remote vehicle). 
     In another embodiment, a method (e.g., for communicating in a vehicle consist) includes receiving a command message from a lead vehicle at a first remote vehicle in the vehicle consist having the lead vehicle, the first remote vehicle, and a second remote vehicle coupled with one another. The command message includes a directive from the lead vehicle for controlling one or more operations of the first remote vehicle and the second remote vehicle. The method also includes transmitting a reply message to the lead vehicle from the first remote vehicle. The reply message includes a first status of the first remote vehicle. The method also includes receiving a second status of the second remote vehicle at the first remote vehicle, storing the second status at the first remote vehicle, and receiving a first retry message transmitted from the lead vehicle at the first remote vehicle. The first retry message includes an identity of the second remote vehicle. The method also includes transmitting a first repeat message from the first remote vehicle to the lead vehicle in response to receiving the first retry message. The first repeat message includes the second status of the second remote vehicle that is stored at the first remote vehicle. 
     In one embodiment, a system (e.g., a communication system) includes a lead communication unit, first and second remote communication units, and a memory. As used herein, the term “unit” includes a hardware and/or software system that operates to perform one or more functions. For example, a unit may include a computer processor, controller, or other logic-based device that performs operations based on instructions stored on a tangible and non-transitory computer readable storage medium, such as a computer memory. Alternatively, a unit may include a hard-wired device that performs operations based on hard-wired logic of the device. The units shown in the attached figures may represent the hardware that operates based on software or hardwired instructions, the software that directs hardware to perform the operations, or a combination thereof. 
     The lead communication unit is configured to be disposed onboard a lead vehicle in a vehicle consist having at least first and second remote vehicles coupled with the lead vehicle. The lead communication unit also is configured to transmit a command message to the remote vehicles for controlling one or more operations of the first and second remote vehicles. The first and second remote communication units are configured to be disposed onboard the first and second remote vehicles of the vehicle consist, respectively. The first and second remote communication units are configured to receive the command message and to broadcast first and second reply messages to the lead vehicle that includes first and second statuses of the first and second remote vehicles, respectively. The lead communication unit is configured to receive the first status of the first remote vehicle and the first remote communication unit is configured to receive the second status of the second remote vehicle. The memory is configured to be disposed onboard the first remote vehicle and to store the second status of the second remote vehicle. The lead communication unit is configured to transmit a retry message that includes an identity of the second remote vehicle when the second reply message to the command message is not received at the lead vehicle from the second remote vehicle. The first remote communication unit is configured to receive the retry message and to transmit a repeat message to the lead vehicle in response to receiving the retry message. The repeat message includes the status of the second remote vehicle. 
     In one another embodiment, a system (e.g., a communication system) includes a lead communication unit and a memory. The lead communication unit is configured to be disposed onboard a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The lead communication unit also is configured to transmit a command message having a directive for controlling one or more operations of the remote vehicles and to receive a reply message from a first remote vehicle of the remote vehicles in the vehicle consist. The reply message transmitted by the first remote vehicle is responsive to the command message and including a status of the first remote vehicle. The memory is configured to be disposed onboard the lead vehicle and to store the status of the first remote vehicle. When a second reply message is not received by the lead communication unit from a second remote vehicle of the remote vehicles in the vehicle consist responsive to transmission of the command message, the lead communication unit is configured to transmit a retry message from the lead vehicle to the remote vehicles that includes an identity of the second remote vehicle and to receive a first repeat message from the first remote vehicle in response to the retry message. The first repeat message includes the status of the second remote vehicle. 
     In another embodiment, a system (e.g., a communication system) includes a first remote communication unit and a memory. The first remote communication unit is configured to be disposed onboard a first remote vehicle of a vehicle consist having a lead vehicle and at least one second remote vehicle coupled with the first remote vehicle. The first remote communication unit is configured to receive a command message from the lead vehicle for controlling one or more operations of the first remote vehicle and the at least one second remote vehicle and to transmit a reply message to the lead vehicle from the first remote vehicle that includes a first status of the first remote vehicle. The memory is configured to be disposed onboard the first remote vehicle. The first remote communication unit is configured to receive a second status of the at least one second remote vehicle that is transmitted by a second remote communication unit responsive to the command message and the memory is configured to store the second status at the first remote vehicle. The first remote communication unit also is configured to receive a first retry message from the lead vehicle that includes an identity of the at least one second remote vehicle and to transmit a first repeat message to the lead vehicle in response to receiving the first retry message. The first repeat message includes the second status of the at least one second remote vehicle that is stored at the first remote vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made briefly to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of one embodiment of a communication system of a vehicle consist; 
         FIGS. 2A and 2B  illustrate a flowchart of one embodiment of a method of communicating in the vehicle consist shown in  FIG. 1 ; 
         FIG. 3  illustrates an example of a timing diagram that is used to demonstrate one example of the method and use of the communication system shown in  FIG. 1 ; and 
         FIG. 4  is a schematic diagram of a powered vehicle in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic view of one embodiment of a communication system  100  of a vehicle consist  102 . The illustrated vehicle consist  102  includes powered vehicles  104 ,  106  (e.g., vehicles  104 ,  106 A,  106 B,  106 C) and non-powered vehicles  108  (e.g., vehicles  108 A,  108 B) mechanically coupled with each other. By “powered,” it is meant that the powered vehicles  104 ,  106  include propulsion subsystems that act to propel the vehicles  104 ,  106  such that the vehicles  104 ,  106  are self-propelled. By “non-powered,” it is meant that the vehicles  108  do not include the propulsion subsystems and are not capable of self-propulsion, but may otherwise receive power (e.g., electric energy) for one or more purposes. The powered vehicles  104 ,  106  are shown as locomotives, the non-powered vehicles  108  are shown as rail cars, and the vehicle consist  102  is shown as a train in the illustrated embodiment. Alternatively, the vehicles  104 ,  106  may represent other vehicles, such as automobiles, marine vessels, or the like, and the vehicle consist  102  can represent a grouping or coupling of these other vehicles. The number and arrangement of the vehicles  104 ,  106  in the vehicle consist  102  are provided as one example and are not intended as limitations on all embodiments of the inventive subject matter described herein. 
     The powered vehicles  104 ,  106  can be arranged in a distributed power (DP) arrangement. For example, the powered vehicles  104 ,  106  can include a lead vehicle  104  that issues command messages to the other powered vehicles  106 A,  106 B,  106 C which are referred to herein as remote vehicles. The designations “lead” and “remote” are not intended to denote spatial locations of the powered vehicles  104 ,  106  in the vehicle consist  102 , but instead are used to indicate which powered vehicle  104 ,  106  is transmitting command messages and which powered vehicles  104 ,  106  are being remotely controlled using the command messages. For example, the lead vehicle  104  may or may not be disposed at the front end of the vehicle consist  102  (e.g., along a direction of travel of the vehicle consist  102 ). Additionally, the remote vehicles  106 A-C need not be separated from the lead vehicle  104 . For example, a remote vehicle  106 A-C may be directly coupled with the lead vehicle  104  or may be separated from the lead vehicle  104  by one or more other remote vehicles  106 A-C and/or non-powered vehicles  108 . 
     The command messages may include directives that direct operations of the remote vehicles. These directives can include propulsion commands that direct propulsion subsystems of the remote vehicles to move at a designated speed and/or power level, brake commands that direct the remote vehicles to apply brakes at a designated level, and/or other commands. The lead vehicle  104  issues the command messages to coordinate the tractive efforts and/or braking efforts provided by the powered vehicles  104 ,  106  in order to propel the vehicle consist  102  along a route  110 , such as a track, road, waterway, or the like. 
     The command messages can be communicated using the communication system  100 , as described below. In one embodiment, the command messages are wirelessly communicated using the communication system  100 . Due to various impediments to wireless communication, some command messages may not be received by one or more of the remote vehicles  106 . In order to confirm whether a command message is received by the remote vehicles  106 , the remote vehicles  106  that receive the command message respond by transmitting a reply message. For example, responsive to receiving the command message from the lead vehicle  104 , a remote vehicle  106  may transmit a reply message. 
     The reply message notifies the lead vehicle  104  that the remote vehicle  106  received the command message from the lead vehicle  104 . The reply message can include a status and an identity of the remote vehicle  106  that transmits the reply message. The status can include data in the reply message (e.g., one or more bits or bytes) that represent one or more current operational states of the remote vehicle  106 , such as a currently implemented tractive effort, a currently implemented braking effort, one or more operational errors of the remote vehicle  106 , or the like. The identity can include data in the reply message that represents which remote vehicle  106  transmitted the reply message. For example, the different remote vehicles  106  may be associated with distinct identifiers (e.g., numeric and/or alphanumeric sequences or codes) that can be used by determine which remote vehicle  106  sent the reply message. 
     The reply messages may be broadcast by the remote vehicles  106  such that the lead vehicle  104  and/or one or more of the other remote vehicles  106  receive the reply messages. For example, the remote vehicle  106 C may transmit a reply message that is received by the lead vehicle  104 , the remote vehicle  106 A and/or  106 B. The remote vehicles  106  that receive reply messages from one or more other remote vehicles  106  may store (e.g., record, log, or otherwise retain in an onboard memory) the status and identity of the other remote vehicles  106 . With respect to the preceding example, one or more of the remote vehicles  106 A and/or  106 B may locally store the status and identity of the remote vehicle  106 C, as described in more detail below. 
     In one embodiment, subsequent to transmitting a command message, the lead vehicle  104  receives one or more reply messages from the remote vehicles  106 . The lead vehicle  104  determines which remote vehicles  106  that the lead vehicle  104  did not receive a reply message from. For example, the lead vehicle  104  may transmit a command message and receive reply messages from the remote vehicles  106 A and  106 C, but not from the remote vehicle  106 B. The lead vehicle  104  can examine the identities in the reply messages that are received at the lead vehicle  104  in order to determine which remote vehicles  106  did not have reply messages received at the lead vehicle  104 . The lead vehicle  104  can compare the identities in the received reply messages with a list, table, or other memory structure that includes the identities of the remote vehicles  106  in the vehicle consist  102  to determine which remote vehicles  106  did not have reply messages received by the lead vehicle  104 . 
     When the lead vehicle  104  determines that reply messages are not received from one or more remote vehicles  106 , the lead vehicle  104  transmits a retry message to the remote vehicles  106 . The retry message can include the directive of the command message and one or more missing reply identifiers. The missing reply identifiers can include the identities of the one or more remote vehicles  106  that did not have reply messages received at the lead vehicle  104 . For example, if the lead vehicle  104  did not receive a reply message from the remote vehicle  106 B, then the retry message may include the identity of the remote vehicle  106 B. 
     The retry message is received by one or more of the remote vehicles  106 . The remote vehicles  106  that receive the retry message can examine the retry message to determine if the one or more missing reply identifiers that are included in the retry message correspond to the identity and associated status of another remote vehicle  106  that is stored onboard the remote vehicle  106  that received the retry message. For example, if a first remote vehicle  106 A stores the identity and status of the second and third remote vehicles  106 B,  106 C, and the lead vehicle  104  transmits a reply message including a missing reply identifier that identifies the second remote vehicle  106 B, then the first remote vehicle  106 A may determine that the lead vehicle  104  did not receive the reply message from the second remote vehicle  106 B. The first remote vehicle  106 A can then transmit the stored identity and status of the second remote vehicle  106 B to the lead vehicle  104  in a repeat message. The repeat message is a message sent by one remote vehicle  106  that includes the identity and status of another remote vehicle  106 . 
     In one embodiment, the lead and remote vehicles  104 ,  106  are assigned non-overlapping time slots during which the vehicles  104 ,  106  are allowed to transmit messages. For example, the lead vehicle  104  may be assigned a periodically repeating first time slot that occurs over a first time period, the first remote vehicle  106 A is assigned a periodically repeating second time slot that occurs over a subsequent, second time period, the second remote vehicle  106 B can be assigned a periodically repeating third time slot that occurs over a subsequent, third time slot, and so on. The vehicles  104 ,  106  may only transmit messages during the time slot assigned to each vehicle  104 ,  106 . When a remote vehicle  106  transmits the status of another remote vehicle  106  in response to receiving a retry message from the lead vehicle  104 , the remote vehicle  106  may transmit during its assigned time slot the status of the other remote vehicle  106  (whose status was stored onboard the remote vehicle  106  that transmits the status to the lead vehicle  104 ). 
     If the lead vehicle  104  receives the identity and status of the second remote vehicle  106 B from the transmission from the first remote vehicle  106 A, then the lead vehicle  104  may remove the second remote vehicle  106 B from the memory structure on the lead vehicle  104  that records which remote vehicles  106  that the lead vehicle  104  has not yet received reply messages since the command message was sent. The lead vehicle  104  may repeatedly transmit retry messages following the transmission of the command message until the lead vehicle  104  receives reply messages or relayed messages from all the remote vehicles  106  in the vehicle consist  102  in one embodiment. Alternatively, the lead vehicle  104  may repeatedly transmit the retry messages following transmission of the command message until the lead vehicle  104  receives reply messages or relayed messages from at least a previously designated number, fraction, or percentage of the total number of remote vehicles  104  in the vehicle consist  102 . 
     In one embodiment, the lead vehicle  104  may transmit the retry message only a previously designated number of times before declaring a communication error. When a communication error occurs, the vehicle consist  102  may enter a communication error state and alert the operator of the lead vehicle  104  that communication has been lost with one or more remote vehicles  106 . In response to this loss of communication alert, the operator may elect to slow or stop movement of the vehicle consist  102 . Alternatively, the lead vehicle  104  may issue a new command message to the remote vehicles  106  when the communication error is declared. 
       FIGS. 2A and 2B  illustrate a flowchart of one embodiment of a method  200  of communicating in the vehicle consist  102  shown in  FIG. 1 . The method  200  may be used in conjunction with one or more embodiments of the communication system  100  shown in  FIG. 1 . For example, the method  200  may be used to communicate command messages from the lead vehicle  104  (shown in  FIG. 1 ) to the remote vehicles  106  (shown in  FIG. 1 ) and to handle the re-transmission of commands and/or replies to ensure that all or at least a designated number of remote vehicles  106  receive the command messages. While the method  200  is described in connection with the vehicle consist  102  and communication system  100  shown in  FIG. 1 , alternatively, the method  200  may be used with another vehicle consist  102  and/or communication system  100 . 
     With continued reference to the method  200  shown in  FIGS. 2A and 2B ,  FIG. 3  illustrates an example of a timing diagram  300  that is used to demonstrate one example of the method  200  and use of the communication system  100 . The timing diagram  300  is shown alongside a horizontal axis  302  that is representative of time. The units and values of time shown for the horizontal axis  302  are provided merely as examples and are not intended to be limiting on all embodiments of the inventive subject matter described herein. A vertical axis  304  represents the different powered vehicles  104 ,  106  of the vehicle consist  100 . For example, the events shown to the right of the corresponding vehicle  104 ,  106  in  FIG. 3  occur at the time periods indicated by the horizontal axis  302 . While the description herein focuses only on the lead vehicle  104  and the first, second, and third remote vehicles  106 A,  106 B,  106 C, the description may apply to a smaller or larger number of remote vehicles  106 . 
     With respect to the method  200  shown in  FIG. 2A , at  202 , the powered vehicles  104 ,  106  continue to operate to propel the vehicle consist  102  based on existing or previously communicated command message. For example, the remote vehicles  106  may continue to operate to propel the vehicle consist  102  based on a previously issue directive sent from the lead vehicle  104 . 
     At  204  (shown in  FIG. 2A ), a command message  306  (shown in  FIG. 3 ) is transmitted by the lead vehicle  104  to the remote vehicles  106 . As described above, the command message  306  may include a directive to the remote vehicles  106  to change tractive efforts and/or braking efforts supplied by the remote vehicles  106 . In the embodiment shown in  FIG. 3 , a communication unit (e.g., communication unit  410  shown in  FIG. 4 ) onboard the lead vehicle  104  is activated during a first activation time period  310  and transmits the command message  306 . 
     With respect to the method  200  shown in  FIG. 2A , at  206 , a determination is made as to whether the command message  306  is received at a remote vehicle  106 . For example, the remote vehicles  106  may determine whether the command message  306  transmitted from the lead vehicle  104  is received. The remote vehicles  106  may determine that the command message  306  is received when the command message  306  is successfully received by the remote vehicles  106  (e.g., the entire command message  306  or at least enough of the command message  306  is received to permit the remote vehicle  106  to follow the directive contained in the command message  306 ). As a result, flow of the method  200  can proceed to  208 . If no command message  306  is received or an insufficient amount of the command message  306  is received for the remote vehicle  106  to be able to understand and implement the directive contained in the command message  306 , then the remote vehicles  106  may determine that no command message  306  is received. As a result, flow of the method  200  may return to  202 . For example, the remote vehicles  106  may continue to operate and wait for a new command message  306  from the lead vehicle  104 . 
     At  208  (shown in  FIG. 2A ), a reply message  316  (shown in  FIG. 3 ) is transmitted by the remote vehicles  106  that received the command message  306 . As described above, the reply message  316  can include an identity of the remote vehicle  106  that is transmitting the reply message  316  and a status of the remote vehicle  106 . Additionally, in one embodiment, the remote vehicle  106  may re-transmit the command message  306  that is received from the lead vehicle  104  as a re-transmitted command message  314  (shown in  FIG. 3 ). The remote vehicles  106  may re-transmit the command message  306  in order to relay the command message  306  from the lead vehicle  104  among the remote vehicles  106  distributed along the length of the vehicle consist  102 . In the illustrated example, the first remote vehicle  106 A can include a communication unit (e.g., communication unit  410  shown in  FIG. 4 ) that is activated during an activation time period  312  (shown in  FIG. 3 ) and then communicates the re-transmitted command message  314  and the reply message  316  after receiving the command message  306 . The designation “S 1 ” in the reply message  316  shown in  FIG. 3  represents the status and identifier of the reply message  316  sent by the first remote vehicle  106 A. Alternatively, the first remote vehicle  106 A may not transmit the re-transmitted command message  314 . 
     At  210  (shown in  FIG. 2A ), the lead vehicle  104  receives the reply message  316  from one or more of the remote vehicles  106 . For example, the communication unit  410  of the lead vehicle  104  can determine whether reply messages  316  sent by one or more of the remote vehicles  106  has been received during a designated time period following transmission of the command message  306 . The lead vehicle  104  can examine reply messages  316  that are received from the remote vehicles  106  and extract the identifiers from the received reply messages  316  (or, if the lead vehicle  104  does not detect receipt of any reply messages  316  during the designated time period, then the lead vehicle  104  may determine that no reply messages  316  have been received). The lead vehicle  104  can compare the extracted identities to a list, table, database, or other memory structure stored onboard the lead vehicle  104  (e.g., in a memory  412  shown in  FIG. 4 ) and determine which remote vehicles  106  are not identified by the received reply messages  316 . 
     With respect to the example shown in  FIG. 3 , each of the second and third remote vehicles  106 B,  106 C that received the command message  306  from the lead vehicle  104  activated respective communication units  410  (shown in  FIG. 4 ) of the remote vehicles  106 B,  106 C during activation time periods  348 ,  350 . These communication units  410  re-transmit the command message  306  from the lead vehicle  104  as re-transmitted command messages  318 ,  320  and transmit reply messages  322 ,  324 . The reply messages  322 ,  324  shown in  FIG. 3  include designations “S 2 ” and “S 3 ” that represent the status and identity of each of the respective second and third remote vehicles  106 B,  106 C. 
     In the illustrated example of  FIG. 3 , the reply message  316  sent by the first remote vehicle  106 A is received by the lead vehicle  104 , but the reply messages  322 ,  324  sent by the second and third remote vehicles  106 B,  106 C, respectively, are not received by the lead vehicle  104 , as indicated by the “X” shown over each of the reply messages  322 ,  324 . As a result, at  210 , the lead vehicle  104  determines that the reply message  316  from the first remote vehicle  106 A is received, but not the reply messages  322 ,  324  from the second and third remote vehicles  106 B,  106 C. Consequently, the status of the first remote vehicle  106 A is acquired by the lead vehicle  104 , but not the statuses of the second or third remote vehicles  106 B,  106 C. 
     The flow of the method  200  shown in  FIG. 2A  may then proceed to  212 , where the status and identity of the first remote vehicle  106 A is stored. For example, the status and identity of the first remote vehicle  106 A may be stored in an onboard memory (e.g., the memory  412 ) of the lead vehicle  104 . Because the reply messages  322 ,  324  from the second and third remote vehicles  106 B,  106 C are not received by the lead vehicle  104 , the statuses of the second and third remote vehicles  106 B,  106 C are not stored or recorded by the lead vehicle  104 . 
     In one embodiment, the reply messages  316 ,  322 ,  324  are broadcast by the remote vehicles  106  such that one or more of the other remote vehicles  106  receive the reply message  316 ,  322 ,  324  that is transmitted in response to the command message  306 . For example, instead of transmitting the reply message  316 ,  322 ,  324  so that only the lead vehicle  104  can receive the reply message  316 ,  322 ,  324 , the remote vehicles  106  may transmit the reply messages  316 ,  322 ,  324  so that other remote vehicles  106  can receive the reply messages  316 ,  322 ,  324 . A remote vehicle  106  that receives the reply message  316 ,  322 ,  324  of another remote vehicle  106  also may store the status and identity contained in the reply message  316 ,  322 ,  324 . For example, the first remote vehicle  106 A can receive the reply messages  322 ,  324  broadcast by the second and/or third remote vehicles  106 B,  106 C, the second remote vehicle  106 B can receive the reply messages  316 ,  324  broadcast by the first and/or third remote vehicles  106 A,  106 C, and/or the third remote vehicle  106 C can receive the reply messages  316 ,  322  broadcast by the first and/or second remote vehicles  106 A,  106 B. The remote vehicles  106  that receive reply messages  316 ,  322 ,  324  transmitted by other remote vehicles  106  can store the statuses and identities included in the received reply messages  316 ,  322 ,  324 . For example, the remote vehicles  106  can locally store the statuses and identities of the other remote vehicles  106  in the memory  412  that is onboard the remote vehicles  106 . 
     At  214 , a determination is made as to whether reply messages  316 ,  322 ,  324  are received by the lead vehicle  104  from all of the remote vehicles  106 . If the reply messages  316 ,  322 ,  324  are received from all of the remote vehicles  106 , then the lead vehicle  104  has the status and identities of the remote vehicles  106  and may not need to re-transmit the command message  306 . As a result, flow of the method  200  can return to  202 . On the other hand, if the reply messages  316 ,  322 ,  324  are not received from all of the remote vehicles  106 , then the lead vehicle  104  may not have confirmation that all of the remote vehicles  106  received the command message  306 . The lead vehicle  104  may need to re-transmit the command message  306  or at least the directive contained in the command message  306 . As a result, flow of the method  200  continues to  216  in  FIG. 2B . 
     Alternatively, a determination can be made at  214  as to whether reply messages  316 ,  322 ,  324  are received at the lead vehicle  104  from at least a designated number, fraction, or percentage of the remote vehicles  106 , but not necessarily all of the remote vehicles  106 . For example, for one or more command messages  306 , the lead vehicle  104  may only need a designated number of and/or certain ones of the remote vehicles  106  to receive the command message  306 , but not all of the remote vehicles  106 . If the reply messages  316 ,  322 ,  324  are received from at least the designated number of the remote vehicles  106 , then the lead vehicle  104  has the status and identities of the remote vehicles  106  and may not need to re-transmit the command message  306 . As a result, flow of the method  200  can return to  202 . On the other hand, if the reply messages  316 ,  322 ,  324  are not received from at least the designated number of remote vehicles  106 , then the lead vehicle  104  may not have confirmation that a sufficient number of the remote vehicles  106  received the command message  306 . The lead vehicle  104  may need to re-transmit the command message  306  or at least the directive contained in the command message  306 . As a result, flow of the method  200  continues to  216  in  FIG. 2B . 
     At  216 , a list is created of the remote vehicles  106  from which the lead vehicle  104  did not receive reply messages  316 ,  322 ,  324  in response to the command message  306 . The term “list” can include a sequence, table, database, or other memory structure that organizes information for later retrieval and/or updating. In one embodiment, the lead vehicle  104  compares the identities of the remote vehicles  106  from which the lead vehicle  104  received reply messages  316 ,  322 ,  324  to a list of the identities of the remote vehicles  104  in the vehicle consist  102 . Based on this comparison, the lead vehicle  104  can determine which remote vehicles  106  have not transmitted reply messages  316 ,  322 ,  324  or which remote vehicles  106  that the lead vehicle  104  has not received the reply message  316 ,  322 ,  324 . These remote vehicles  106  can be referred to as “missing remote vehicles  106 .” The list of missing remote vehicles  106  can be created and stored onboard the lead vehicle  104 , such as on the memory  412  of the lead vehicle  104 . Alternatively, the list may be created and/or stored at an off-board location (e.g., a dispatch or other facility) and communicated to the lead vehicle  104 . In the example shown in  FIG. 3 , the lead vehicle  104  creates a list having the identities of the second and third remote vehicles  106 B,  106 C, as the lead vehicle  104  has not received a reply message  316 ,  322 ,  324  from the second or third remote vehicles  106 B,  106 C. 
     At  218  (shown in  FIG. 2B ), a retry message is transmitted from the lead vehicle  104 . As described above, the retry message can be the same as or similar to the command message  306 . For example, the retry message can include the directive that previously was transmitted in the command message  306 . The lead vehicle  104  transmits the retry message due to the failure of one or more (or more than a designated number) of the remote vehicles  106  to respond to the previously sent command message  306 . 
     As shown in  FIG. 3 , a retry message  326  can be transmitted by the lead vehicle  104 , such as by the communication unit  410  of the lead vehicle  104 . The communication unit  410  may be activated during an activation time period  328  and then the communication unit  410  may wirelessly transmit the retry message  326  and/or transmit the retry message  326  through one or more wired connections. In one embodiment, the retry message  326  includes the directive previously sent in the command message  306  and also includes the identities of one or more of the missing remote vehicles  106 . For example, the retry message  326  can include the identities of the remote vehicles  106  from which a reply message  316 ,  322 ,  324  was not received responsive to the previous transmission of the command message  306 . The designation “R 2 ,  3 ” in the retry message  326  in  FIG. 3  indicates that the identities of the second and third remote vehicles  106 B,  106 C are included in the retry message  326 . These identities can be included in the retry message  326  so as to notify the remote vehicles  106  that the lead vehicle  104  has not received reply messages  316 ,  322 ,  324  from the remote vehicles  106  associated with the identities. 
     At  220  (shown in  FIG. 2 ), a determination is made as to whether the retry message  326  is received at the remote vehicles  106 . For example, the communication units  410  of the remote vehicles  106  may receive the retry message  326  from the lead vehicle  106 . Alternatively, the communication units  410  of the remote vehicles  106  may determine that the retry message  326  is not received if no retry message  326  is received within a designated time period. 
     If the retry message  326  is received at a remote vehicle  106 , then the remote vehicle  106  can examine the retry message  326  to determine if the remote vehicle  106  can provide the lead vehicle  104  with the status and identity of one or more of the remote vehicles  106  from which the lead vehicle  104  did not receive a reply message  316 ,  322 ,  324 . As a result, flow of the method  200  may continue to  222 . Otherwise, the method  200  may return to  218  if the retry message  326  is not received at the remote vehicle  106 . For example, the lead vehicle  104  can re-send the retry message  326  after a designated time period of sending a previous retry message  326 . 
     A remote vehicle  106  that receives the retry message  326  (e.g., a “receiving remote vehicle  106 ”) examines the identities of the missing remote vehicles  106  included in the retry message  326 . The receiving remote vehicle  106  can extract the identities of the missing remote vehicles  106  included in the retry message  326  to determine which of remote vehicles  106  that the lead vehicle  104  does not have the status. The receiving remote vehicle  106  can compare the identities of the missing remote vehicles  106  in the retry message  326  and compare these identities to the identities of the remote vehicles  106  that are stored by the receiving remote vehicle  106 . For example, the receiving remote vehicle  106  can compare the identities in the retry message  326  with the identities stored in the memory  412  of the receiving remote vehicle  106 . As described above, the identities stored in the memory  412  can include the identities of the remote vehicles  106  whose reply messages  316  were received by the receiving remote vehicle  106 . 
     At  222 , a determination is made as to whether a remote vehicle  106  that receives the retry message  326  is associated with an identity of a missing remote vehicle  106  included in the retry message  326 . With respect to the example of  FIG. 3 , if the second or third remote vehicles  106 B,  106 C receive the retry message  326  with the identity of the second or third remote vehicle  106 B,  106 C, then the second or third remote vehicle  106 B,  106 C may determine that the remote vehicle  106 B or  106 C can respond with a status of the vehicle  106 B or  106 C. As a result, flow of the method  220  may continue to  224 . On the other hand, if the statuses of the missing remote vehicles  106  in the retry message  326  do not match the identity of the remote vehicle  106  that receives the retry message  326 , then the remote vehicle  106  may not respond with the status of the remote vehicle  106 . As a result, flow of the method  200  may continue to  226 . 
     At  224 , the remote vehicle  106  that received the retry message  326  and that has the identity of the receiving remote vehicle  106  as a missing remote vehicle  106  can transmit a reply message to the lead vehicle  104 . This reply message can include the identity and status of the receiving remote vehicle  106 , similar to the reply message  316  described above. 
     At  226 , a determination is made as to whether one or more of the identities of the missing remote vehicles  106  that are included in the retry message  326  match the identities stored in the memory  412  of the receiving remote vehicle  106 . For example, in addition to or in place of determining whether the identity of the receiving remote vehicle  106  matches the identities of the missing remote vehicles  106  in the retry message  326 , the receiving remote vehicle  106  may determine whether any of the statuses and identities stored onboard the receiving remote vehicle  106  match the identities in the retry message  326 . As described above, the receiving remote vehicle  106  may locally store identities and statuses of other remote vehicles  106  based on reply messages  316 ,  322 ,  324  that are received by the receiving remote vehicle  106 . If the identities of the missing remote vehicles  106  in the retry message  326  match the stored identities, then the receiving remote vehicle  106  may respond with the missing statuses and identities. As a result, flow of the method  200  may continue to  228 . On the other hand, if the identifies of the missing remote vehicles  106  in the retry message  326  do not match the stored identifies, then the receiving remote vehicle  106  may not respond to the retry message  326 . As a result, flow of the method  200  may return to  218 . For example, the communication unit  410  of the remote vehicle  106  may wait for additional retry messages  326  to be sent by the lead vehicle  104 . Alternatively or additionally, flow of the method  200  may return to  202  to wait for additional command messages  306  from the lead vehicle  104  or to  210  to wait for the receipt of reply messages  316 ,  322 ,  324  from other remote vehicles  106 . 
     At  228 , the receiving remote vehicle  106  transmits a repeat message to the lead vehicle  104  that includes the status and identity of at least one of the remote vehicles  106  identified by the retry message  326 . With respect to the example shown in  FIG. 3 , the first remote vehicle  106 A may receive the retry message  326  that includes the identities of the missing remote vehicles  106 B,  106 C. The first remote vehicle  106 A compares these identities to the identities stored onboard the first remote vehicle  106 A and determines that the first remote vehicle  106 A has stored the statuses of the second and third remote vehicles  106 B,  106 C (based on previously receiving the reply messages  322 ,  324  from the second and third remote vehicles  106 B,  106 C, as described above). In response, the first remote vehicle  106 A transmits a repeat message to the lead vehicle  104  that includes the status of the second and/or third remote vehicle  106 B,  106 C. 
     For example, as shown in  FIG. 3 , during an activation time period  332 , the communication unit  410  of the first remote vehicle  106 A turns on or is otherwise activated. The communication unit  410  of the first remote vehicle  106 A receives the retry message  326  from the lead vehicle  106 A. The first remote vehicle  106 A then re-transmits the lead retry message  334  and transmits a repeat message  330  to the lead vehicle  104  that includes the status of the second remote vehicle  106 B (as shown by the designation “S 2 ” in  FIG. 3 ). 
     In one embodiment, a remote vehicle  106  responds to the retry message  326  with the status of a single remote vehicle  106 . For example, even though the first remote vehicle  106 A has the statuses of both the second and third remote vehicles  106 B,  106 C stored onboard the first remote vehicle  106 A, the first remote vehicle  106 A responds to the retry message  326  with the stored status of the second remote vehicle  106 B and not the status of any other remote vehicle  106 . The remote vehicles  106  may respond to retry messages  326  or  334  with the status of a designated remote vehicle  106 , such as the next remote vehicle  106  along the length of the vehicle consist  102 . For example, the first remote vehicle  106 A can respond with the status of the second remote vehicle  106 B, the second remote vehicle  106 B can respond with the status of the third remote vehicle  106 C, and so on. Alternatively, the remote vehicles  106 A may respond to a retry message  326  with several stored statuses of remote vehicles  106  that are identified by the retry message  326 . 
     After transmitting the repeat message  330  with the status of one or more remote vehicles  106  that are identified in the retry message  326  from the lead vehicle  104 , the remote vehicle  106  that transmitted the repeat message  330  may remove (e.g., delete) the status of the remote vehicle  106  identified by the retry message  326  from the onboard memory  412 . For example, after the first remote vehicle  106 A transmits the repeat message  330  with the stored status of the second remote vehicle  106 B, the first remote vehicle  106 A may delete the status of the second remote vehicle  106 B from the memory  412  of the first remote vehicle  106 A. 
     With respect to the third remote vehicle  106 C in the example shown in  FIG. 3 , the third remote vehicle  106 C may receive the retry message  326  or  334  that identifies the third remote vehicle  106 C as a missing remote vehicle  106 . As a result, the communication unit  410  of the third remote vehicle  106 C activates during an activation time period  336  and transmits a reply message  338 . As shown by the designation “S 3 ” in  FIG. 3 , the reply message  338  includes the status of the third remote vehicle  106 C. 
     In the example of  FIG. 3 , the lead vehicle  104  receives the repeat message  330  from the first remote vehicle  106 A that includes the status of the second remote vehicle  106 B, as described above. The lead vehicle  104  does not, however, receive the reply message  338  transmitted by the third remote vehicle  106 C, as shown by the “X” drawn over the reply message  338 . After receiving the repeat message  330  from the first remote vehicle  106 A, the communication unit  410  of the lead vehicle  104  may activate during an activation time period  342  and transmit another retry message  340  that includes the identity of the third remote vehicle  106 C. The retry message  340  includes the identity of the third remote vehicle  106 C because the lead vehicle  104  still does not have the status of the third remote vehicle  106 C. 
     In the illustrated example, the first remote vehicle  106 A receives the second retry message  340  and extracts the identity of the third remote vehicle  106 C from the retry message  340 . The first remote vehicle  106 A examines the identities and statuses stored in the memory  412  of the first remote vehicle  106 A (as described above), and determines that the first remote vehicle  106 A has the status of the third remote vehicle  106 C. For example, the communication unit  410  of the first remote vehicle  106 A activates during an activation time period  344  and transmits a repeat message  346  to the lead vehicle  104  that includes the status of the third remote vehicle  106 C. When the lead vehicle  104  receives the repeat message  346 , the lead vehicle  104  has the statuses of the remote vehicles  106 A,  106 B,  106 C. 
     While the description of the illustrated examples focuses on the first remote vehicle  106 A transmitting the statuses of the second and third remote vehicles  106 B,  106 C in the repeat messages  330 ,  346  to the lead vehicle  104 , the description also may apply to the second, third, or other remote vehicle  106  in the vehicle consist  102 . For example, the second remote vehicle  106 B may transmit the statuses of the first, third, or other remote vehicle  106  in repeat messages when the second remote vehicle  106 B receives retry messages from the lead vehicle  104 . 
     In one embodiment of implementing the method  200  shown in  FIGS. 2A and 2B  with the system  100 , the lead vehicle  104  transmits the command message  306  to the remote vehicles  106 . The first remote vehicle  106 A receives the command message  306  and transmits the status of the first remote vehicle  106 A to the lead vehicle  104  in the reply message  316 . The first remote vehicle  106 A also repeats the command message  306  by transmitting the command message  314 . The second remote vehicle  106 B receives the command message  306  and/or  314  and transmits the status of the second remote vehicle  106 B to the lead vehicle  104  in the reply message  322 . The second remote vehicle  106 B also repeats the command message  306  or  314  by transmitting another re-transmitted command message  318 . The third remote vehicle  106 C receives the command message  306  and/or  314  and/or  318  and transmits the status of the third remote vehicle  106 C to the lead vehicle  104  in the reply message  324 . The third remote vehicle  106 C also can repeat the command message  306  or  314  or  318  by transmitting another re-transmitted command message  320 . 
     One or more of the remote vehicles  106  may receive the reply message  316 ,  322 ,  324  sent by one or more other remote vehicles  106 . The remote vehicles  106  that receive the reply messages  316 ,  322 ,  324  can locally store the identity and associated status included in the reply messages  316 ,  322 ,  324 . In one embodiment, each remote vehicle  106  may only store the status and identity of designated ones of the remote vehicles  106 . For example, a remote vehicle  106  may only store the statuses and identities of those remote vehicles  106  that are downstream of the remote vehicle  106  along a direction of travel of the vehicle consist  102 . Alternatively, the remote vehicles  106  may store the statuses and identities of other remote vehicles  106 . 
     In one embodiment, if the lead vehicle  104  does not receive the reply messages  316 ,  322 ,  324  from each remote vehicle  106 , then the lead vehicle  104  re-transmits the command message (e.g., in the retry message  326 ,  340 ). The lead vehicle  104  may delay re-transmission of the command message by at least a designated amount to avoid interfering with the transmission of reply messages  316 ,  322 ,  324  by the remote vehicles  106 . This delay may be based on the number of remote vehicles  106  from which the lead vehicle  104  has not received a status and identity. For example, the communication unit  410  of the lead vehicle  104  may delay re-transmission of the command message by a time period multiplied or otherwise increased by the number of remote vehicles  106  from which the lead vehicle  104  has not yet received a reply message  316 ,  322 ,  324 . In one embodiment, the time delay is 512 msec times the number of remote vehicles  106  from which the lead vehicle  104  has not yet received a reply message  316 ,  322 ,  324 . 
     The retry message  326  that is transmitted by the communication unit  410  of the lead vehicle  104  may be modified from the previously transmitted command message  306 . For example, the retry message  326  may include one or more bits or bytes that are not included in the command message  306  that indicate that the retry message  326  is a retry message and not another command message. Such a bit or byte may be referred to as a “Retry Command/Repeat Status” byte. The remote vehicles  106  that receive the retry message  326  may identify this bit or byte in order to differentiate between new command messages  306  and retry messages  326 . This bit or byte (or another bit or byte) in the retry message  326  may include an identifier bit or byte that indicates the remote vehicles  106  from which the lead vehicle  104  has not received reply messages  316 ,  322 ,  324 . 
     In one embodiment, as each remote vehicle  106  receives the retry message  326  with the identified missing remote statuses, the receiving remote vehicle  106  transmits the status of the lowest missing remote vehicle  106  from the list of locally stored statuses and identities. By “lowest missing remote vehicle  106 ,” it is meant that the remote vehicle  106  that receives the retry message  326  transmits the status of the remote vehicle  106  that is closest to the receiving remote vehicle  106  along the length of the vehicle consist  102 , such as the next remote vehicle  106  located downstream from the receiving remote vehicle  106  along the length of the vehicle consist  102  in the direction of travel of the vehicle consist  102 . Alternatively, the receiving remote vehicle  106  may transmit the status of another remote vehicle  106 . The remote vehicles  106  may be assigned non-overlapping time slots during which the remote vehicles  106  are to transmit messages to the lead vehicle  104 . The remote vehicle  106  that receives the retry message  326  may transmit the status of another remote vehicle  106  in the time slot that is assigned to the receiving remote vehicle  106 . Alternatively, the remote vehicle  106  may transmit the status during another time slot. 
     As each remote vehicle  106  transmits the status of another remote vehicle  106  to the lead vehicle  104 , the remote vehicle  106  that transmitted the status will remove the status of the other remote vehicle  106  from the stored list of statuses that is onboard the remote vehicle  106  that transmitted the status. 
     The communication unit  410  of the lead vehicle  104  may wait between transmissions of command messages  306  and retry messages  326  to receive the reply messages  316 ,  322 ,  324  and/or repeat messages  330  sent by the remote vehicles  106 . In one embodiment, if the communication unit  410  of the lead vehicle  104  has not received the statuses of all of the remote vehicles  106  in the vehicle consist  102  after transmitting the retry message  326  a number of times that is equivalent to the number of remote vehicles  104  in the vehicle consist  102 , then the lead vehicle  104  may cease sending additional retry messages  326 . For example, the lead vehicle  104  may send another command message  306  that does not include the “Retry Command /Repeat Status” bit or byte. The remote vehicles  106  that receive this new command message  306  can then respond by transmitting a reply message  316 ,  322 ,  324  with the status of the remote vehicles  106  and may begin updating the locally stored list of statuses of other remote vehicles  106  (based on the reply messages  316 ,  322 ,  324  received by the other remote vehicles  106 ). 
     During the transmission of retry messages  326  to the remote vehicles  106 , the directives sent to the remote vehicles  106  from the lead vehicle  104  may need to change. For example, the tractive efforts, braking efforts, speed, power output, or the like, that is automatically or manually demanded from the remote vehicles  106  may change. The lead vehicle  104  can transmit another command message  306  that does not include the “Retry Command/Repeat Status” bit or byte. The remote vehicles  106  that receive this new command message  306  can then respond by transmitting a reply message  316 ,  322 ,  324  with the status of the remote vehicles  106  and may begin updating the locally stored list of statuses of other remote vehicles  106  (based on the reply messages  316 ,  322 ,  324  received by the other remote vehicles  106 ). 
     Once the lead vehicle  104  has received the statuses of all of the remote vehicles  106 , in one embodiment, the lead vehicle  104  can return to a normal periodic transmission of the command messages  306 . For example, if, prior to the transmission of the retry messages  326 , the lead vehicle  104  was transmitting a new command message  306  (with the same or different directives for the remote vehicles  106 ) every 20 seconds (or other time period), then the lead vehicle  104  may return to this periodic transmission of command messages  306  after receiving the statuses of all of the remote vehicles  10  in one embodiment. 
       FIG. 4  is a schematic diagram of a powered vehicle  400  in accordance with one embodiment. The powered vehicle  400  may represent one or more of the powered vehicles  104 ,  106  shown in  FIG. 1 . The powered vehicle  400  includes a control unit  402  that controls operations of the powered vehicle  400 . The control unit  402  is connected with an input device  404  and an output device  406 . The control unit  402  can receive manual input from an operator of the powered vehicle  400  through the input device  404 , such as a touchscreen, keyboard, electronic mouse, microphone, or the like. For example, the control unit  402  can receive manually input changes to the tractive effort, braking effort, speed, power output, and the like, from the input device  404 . The control unit  402  can present information to the operator using the output device  406 , which can represent a display screen (e.g., touchscreen or other screen), speakers, printer, or the like. For example, the control unit  402  can present the identities and statuses of the remote vehicles  106 , identities of the missing remote vehicles  106  (e.g., those remote vehicles  106  from which the lead vehicle  104  has not received the status), contents of one or more command messages, retry messages, reply messages, repeat messages, or the like. 
     The control unit  402  is connected with a propulsion subsystem  408  of the powered vehicle  400 . The propulsion subsystem  408  provides tractive effort and/or braking effort of the powered vehicle  400 . The propulsion subsystem  408  may include or represent one or more engines, motors, alternators, generators, brakes, batteries, turbines, and the like, that operate to propel the powered vehicle  400  under the manual or autonomous control that is implemented by the control unit  400 . For example, the control unit  400  can generate control signals autonomously or based on manual input that is used to direct operations of the propulsion subsystem  408 . 
     The control unit  402  also is connected with the communication unit  410  and the memory  412  of the powered vehicle  400 . The memory  412  can represent an onboard device that electronically and/or magnetically stores data. For example, the memory  412  may represent a computer hard drive, random access memory, read-only memory, dynamic random access memory, an optical drive, or the like. 
     The communication unit  410  includes or represents hardware and/or software that is used to communicate with other powered vehicles  400  in the vehicle consist  102 . For example, the communication unit  410  may include a transceiver  414  and associated circuitry for wirelessly communicating (e.g., transmitting and/or receiving) command messages, reply messages, retry messages, and/or repeat messages, as described above. Additionally or alternatively, the communication unit  410  include circuitry for communicating command messages, reply messages, retry messages, and/or repeat messages over a wired connection  416 , such as an electric multiple unit (eMU) line of the vehicle consist  102  or another conductive pathway between or among the powered vehicles  104 ,  106 ,  400  in the vehicle consist  102 . The control unit  402  may control the communication unit  410  by activating the communication unit  410  (as described above). The communication unit  410  can examine the messages that are received by the powered unit  400  as described above. For example, the communication unit  410  of a remote vehicle  106  can examine received command messages to determine the directive sent by the lead vehicle  104 . The directive can be conveyed to the control unit  402 , which then implements the directive by creating control signals that are communicated to the propulsion subsystem  408  for autonomous control or by presenting the directive to the operator on the output device  406  for manual implementation of the directive. 
     The communication unit  410  of a remote vehicle  106  can examine received reply messages sent by other remote vehicles  106  to determine the identities and statuses of the other remote vehicles  106 , as described above. The communication unit  410  can store these received identities and statuses in the memory  412 . The communication unit  410  of a remote vehicle  106  can receive and examine retry messages sent by the lead vehicle  104  and determine if the memory  412  has the statuses stored of the missing remote vehicles  106  that are identified in the retry messages, as described above. The communication unit  410  can transmit the repeat messages to provide the lead vehicle  104  with the stored statuses. The communication unit  410  of the remote vehicles  104  also can re-transmit the command messages received from the lead vehicle  104 , as described above. 
     The communication unit  410  of the lead vehicle  104  can transmit the command messages and determine which remote vehicles  106  have not responded with the statuses of the remote vehicles  106 . The communication unit  410  can then transmit the retry messages that include the identities of the remote vehicles  106  having missing statues, as described above. 
     In one embodiment, a method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in the vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving the command message at the remote vehicles and transmitting reply messages to the lead vehicle from the remote vehicles. The reply messages include statuses of the remote vehicles that transmitted the reply messages. The method further includes receiving the status of a first remote vehicle of the two or more remote vehicles at the lead vehicle and receiving the status of a second remote vehicle of the two or more remote vehicles at the first remote vehicle. The status of the second remote vehicle is not received at the lead vehicle. The method also includes storing the status of the second remote vehicle at the first remote vehicle and transmitting a retry message from the lead vehicle to the remote vehicles. The retry message includes an identity of the second remote vehicle. The method further includes receiving the retry message at the first remote vehicle and transmitting a repeat message from the first remote vehicle to the lead vehicle in response to receiving the retry message. The repeat message includes the status of the second remote vehicle. 
     In one aspect, storing the status of the second remote vehicle includes recording the status in a memory disposed onboard the first remote vehicle. 
     In one aspect, the retry message that is transmitted from the lead vehicle additionally includes the directive previously transmitted in the command message. 
     In one aspect, the method also includes re-transmitting the command message from the first remote vehicle and the second remote vehicle responsive to receiving the command message from the lead vehicle at the first remote vehicle and the second remote vehicle. 
     In one aspect, the directive that is included in the command message directs the remote vehicles to change at least one of moving speeds of the remote vehicles, power outputs of the remote vehicles, tractive efforts provided by propulsion subsystems of the remote vehicles, or braking efforts provided by the propulsion subsystems of the remote vehicles. 
     In one aspect, the statuses that are included in the reply messages represent at least one of current operational states of the remote vehicles, currently implemented tractive efforts provided by the remote vehicles, currently implemented braking efforts provided by the remote vehicles, or operational errors of the remote vehicles. 
     In one aspect, each of the remote vehicles is assigned a time slot during which the remote vehicle communicates with the lead vehicle. Transmitting the repeat message from the first remote vehicle that includes the status of the second remote vehicle can occur during the time slot assigned to the first remote vehicle. The assigned time slots may be non-overlapping time periods. 
     In another embodiment, another method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving a first reply message from a first remote vehicle of the remote vehicles in the vehicle consist. The first reply message is transmitted by the first remote vehicle responsive to the command message and including a status of the first remote vehicle. A second reply message is not received from a second remote vehicle of the remote vehicles in the vehicle consist responsive to the command message. The method further includes transmitting a retry message from the lead vehicle to the remote vehicles. The retry message includes an identity of the second remote vehicle. The method also includes receiving a first repeat message from the first remote vehicle in response to the retry message. The first repeat message includes the status of the second remote vehicle. 
     In another embodiment, another method (e.g., for communicating in a vehicle consist) includes transmitting a command message from a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The command message includes a directive for controlling one or more operations of the remote vehicles. The method also includes receiving a first reply message from a first remote vehicle of the two or more remote vehicles in the vehicle consist responsive to the command message. The first reply message includes a status of the first remote vehicle. If a second reply message is not received from a second remote vehicle of the remote vehicles responsive to the command message, then the method also includes transmitting a retry message from the lead vehicle to the remote vehicles (where the retry message includes an identity of the second remote vehicle) and receiving a first repeat message from the first remote vehicle in response to the retry message (where the first repeat message includes the status of the second remote vehicle). 
     In one aspect, transmitting the retry message is repeated until the status of the second remote vehicle is received or until the retry message is re-transmitted a designated number of times. 
     In one aspect, additional reply messages are not received from a missing set of two or more of the remote vehicles that includes the second remote vehicle, and transmitting the retry message includes transmitting the retry message with identities of each of the remote vehicles in the missing set of the remote vehicles. 
     In one aspect, receiving the first repeat message includes receiving at least one second repeat message from one or more of the remote vehicles. The second repeat message includes the status of one or more of the remote vehicles in the missing set of the remote vehicles. 
     In one aspect, transmitting the retry message is repeated until the statuses of the remote vehicles in the missing set of remote vehicles are received in the first repeat message or the at least one second repeat message. 
     In one aspect, the method also includes, responsive to receiving the statuses of one or more of the remote vehicles in the missing set of remote vehicles, removing the identities of the one or more of the remote vehicles for which the identities are received and transmitting the retry message without the identities that are removed. 
     In one aspect, the directive that is included in the command message directs the remote vehicles to change at least one of moving speeds of the remote vehicles, power outputs of the remote vehicles, tractive efforts provided by propulsion subsystems of the remote vehicles, or braking efforts provided by the propulsion subsystems of the remote vehicles. 
     In one aspect, the status that is included in the second reply message represents at least one of a current operational state of the second remote vehicle, a currently implemented tractive effort provided by the second remote vehicle, a currently implemented braking effort provided by the second remote vehicle, or an operational error of the second remote vehicle. 
     In one aspect, each of the remote vehicles is assigned a time slot during which the remote vehicle communicates with the lead vehicle. The first repeat message that is received from the first remote vehicle and that includes the status of the second remote vehicle is transmitted during the time slot assigned to the first remote vehicle. The time slots assigned to the remote vehicles may be non-overlapping time periods. 
     In another embodiment, a method (e.g., for communicating in a vehicle consist) includes receiving a command message from a lead vehicle at a first remote vehicle in the vehicle consist having the lead vehicle, the first remote vehicle, and a second remote vehicle coupled with one another. The command message includes a directive from the lead vehicle for controlling one or more operations of the first remote vehicle and the second remote vehicle. The method also includes transmitting a reply message to the lead vehicle from the first remote vehicle. The reply message includes a first status of the first remote vehicle. The method also includes receiving a second status of the second remote vehicle at the first remote vehicle, storing the second status at the first remote vehicle, and receiving a first retry message transmitted from the lead vehicle at the first remote vehicle. The first retry message includes an identity of the second remote vehicle. The method also includes transmitting a first repeat message from the first remote vehicle to the lead vehicle in response to receiving the first retry message. The first repeat message includes the second status of the second remote vehicle that is stored at the first remote vehicle. 
     In one aspect, the second status of the second remote vehicle is not received at the lead vehicle prior to receiving the first retry message from the lead vehicle. 
     In one aspect, storing the second status includes recording the second status in a memory disposed onboard the first remote vehicle. 
     In one aspect, the method also includes re-transmitting the command message from the first remote vehicle responsive to receiving the command message from the lead vehicle at the first remote vehicle. 
     In one aspect, the method also includes removing the second status from storage at the first remote vehicle responsive to transmitting the first repeat message that includes the second status. 
     In one aspect, receiving the second status includes receiving one or more additional statuses of one or more additional remote vehicles in the vehicle consist and storing the second status includes storing the one or more additional statuses at the first remote vehicle. 
     In one aspect, the method also includes receiving at least one second retry message including the identity of the one or more additional remote vehicles and transmitting at least one second repeat message that includes the one or more additional statues of the one or more additional remote vehicles responsive to receiving the at least one second retry message. 
     In one aspect, the first remote vehicle and the second remote vehicle are assigned respective time slots to communicate with the lead vehicle. Transmitting the first repeat message that includes the second status of the second remote vehicle from the first remote vehicle can occur during the time period assigned to the first remote vehicle. The time slots assigned to the first and second remote vehicles may represent non-overlapping time periods. 
     In one embodiment, a system (e.g., a communication system) includes a lead communication unit, first and second remote communication units, and a memory. The lead communication unit is configured to be disposed onboard a lead vehicle in a vehicle consist having at least first and second remote vehicles coupled with the lead vehicle. The lead communication unit also is configured to transmit a command message to the remote vehicles for controlling one or more operations of the first and second remote vehicles. The first and second remote communication units are configured to be disposed onboard the first and second remote vehicles of the vehicle consist, respectively. The first and second remote communication units are configured to receive the command message and to broadcast first and second reply messages to the lead vehicle that includes first and second statuses of the first and second remote vehicles, respectively. The lead communication unit is configured to receive the first status of the first remote vehicle and the first remote communication unit is configured to receive the second status of the second remote vehicle. The memory is configured to be disposed onboard the first remote vehicle and to store the second status of the second remote vehicle. The lead communication unit is configured to transmit a retry message that includes an identity of the second remote vehicle when the second reply message to the command message is not received at the lead vehicle from the second remote vehicle. The first remote communication unit is configured to receive the retry message and to transmit a repeat message to the lead vehicle in response to receiving the retry message. The repeat message includes the status of the second remote vehicle. 
     In one aspect, the retry message that is transmitted by the lead communication unit includes the identity of the second remote vehicle and a directive previously transmitted in the command message. 
     In one aspect, the first remote communication unit is configured to re-transmit the command message to the second remote vehicle responsive to receiving the command message from the lead vehicle. 
     In one aspect, the command message directs the first and second remote vehicles to change at least one of moving speeds of the first and second remote vehicles, power outputs of the first and second remote vehicles, tractive efforts provided by propulsion subsystems of the first and second remote vehicles, or braking efforts provided by the propulsion subsystems of the first and second remote vehicles. 
     In one another embodiment, a system (e.g., a communication system) includes a lead communication unit and a memory. The lead communication unit is configured to be disposed onboard a lead vehicle in a vehicle consist having two or more remote vehicles coupled with the lead vehicle. The lead communication unit also is configured to transmit a command message having a directive for controlling one or more operations of the remote vehicles and to receive a reply message from a first remote vehicle of the remote vehicles in the vehicle consist. The reply message transmitted by the first remote vehicle is responsive to the command message and including a status of the first remote vehicle. The memory is configured to be disposed onboard the lead vehicle and to store the status of the first remote vehicle. When a second reply message is not received by the lead communication unit from a second remote vehicle of the remote vehicles in the vehicle consist responsive to transmission of the command message, the lead communication unit is configured to transmit a retry message from the lead vehicle to the remote vehicles that includes an identity of the second remote vehicle and to receive a first repeat message from the first remote vehicle in response to the retry message. The first repeat message includes the status of the second remote vehicle. 
     In one aspect, when a reply message is not received from a missing set of two or more of the remote vehicles that includes the second remote vehicle responsive to the transmission of the command message, the lead communication unit is configured to transmit the retry message with identities of each of the remote vehicles in the missing set of the remote vehicles and the memory is configured to store the identities of the remote vehicles in the missing set. 
     In one aspect, the lead communication unit is configured to receive at least a second repeat message from one or more of the remote vehicles. The second repeat message includes the status of one or more of the remote vehicles in the missing set of the remote vehicles. The memory is configured to remove the identities of the remote vehicles in the missing set for which the statuses are received. 
     In another embodiment, a system (e.g., a communication system) includes a first remote communication unit and a memory. The first remote communication unit is configured to be disposed onboard a first remote vehicle of a vehicle consist having a lead vehicle and at least one second remote vehicle coupled with the first remote vehicle. The first remote communication unit is configured to receive a command message from the lead vehicle for controlling one or more operations of the first remote vehicle and the at least one second remote vehicle and to transmit a reply message to the lead vehicle from the first remote vehicle that includes a first status of the first remote vehicle. The memory is configured to be disposed onboard the first remote vehicle. The first remote communication unit is configured to receive a second status of the at least one second remote vehicle that is transmitted by a second remote communication unit responsive to the command message and the memory is configured to store the second status at the first remote vehicle. The first remote communication unit also is configured to receive a first retry message from the lead vehicle that includes an identity of the at least one second remote vehicle and to transmit a first repeat message to the lead vehicle in response to receiving the first retry message. The first repeat message includes the second status of the at least one second remote vehicle that is stored at the first remote vehicle. 
     In one aspect, the memory is configured to remove the second status from storage at the first remote vehicle responsive to transmitting the first repeat message that includes the second status. 
     In one aspect, the first remote communication unit is configured to receive one or more additional statuses of one or more additional remote vehicles in the vehicle consist and the memory is configured to store the one or more additional statuses at the first remote vehicle. 
     In one aspect, the first remote communication unit is configured to receive at least a second retry message including the identity of the one or more additional remote vehicles, and the first remote communication unit is configured to transmit at least a second repeat message that includes the one or more additional statues of the one or more additional remote vehicles responsive to receiving the at least a second retry message. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the inventive subject matter without departing from its scope. While the dimensions and types of materials described herein are intended to define the parameters of the inventive subject matter, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to one of ordinary skill in the art upon reviewing the above description. The scope of the inventive subject matter should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim  1 imitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
     This written description uses examples to disclose several embodiments of the inventive subject matter and also to enable one of ordinary skill in the art to practice the embodiments of inventive subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the inventive subject matter is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     The foregoing description of certain embodiments of the present inventive subject matter will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. Thus, for example, one or more of the functional blocks (for example, processors or memories) may be implemented in a single piece of hardware (for example, a general purpose signal processor, microcontroller, random access memory, hard disk, and the like). Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, and the like. The various embodiments are not limited to the arrangements and instrumentality shown in the drawings. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present inventive subject matter are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.