Abstract:
The present disclosure describes methods and systems for connecting passive signaling devices (“PSDs”) to a railroad track and using the PSDs to optimize the amplitude, modulation, coding, and frequency of waveforms that applied to the track (by signaling points) for at least three track circuit functions: detecting trains, detecting broken rails, and communicating between signaling points and PSDs.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates to railroads generally, and more particularly, to methods and systems for using passive signaling in jointless track circuits. 
         [0003]    2. Discussion of Related Art 
         [0004]    Conventional track circuits use signaling points to monitor a block of railroad track for the presence of trains and broken rails. Signals transmitted and/or received by the signaling points indicating the block state (e.g., whether occupied, empty, or containing a broken rail) are used to directly control the wayside signal aspects, and to send information to the train (via cab signals in the rail) or a central office (via remote communication links). 
         [0005]    Blocks of railroad track are separated from each other by insulative joints (e.g., pieces of electrically insulative material), which are interposed between sections of rail. Use of jointed tracks, however, has several disadvantages. First, the pieces of electrically insulative material are expensive to install and maintain, and tend to deteriorate over time. Additionally, the distance between signaling points is limited because leakage current flows through the ballast (e.g., the material under and/or between the rails that forms or rests on the railroad bed), thereby attenuating an applied voltage between the rails. The attenuation typically occurs exponentially with distance from the source signaling point. 
         [0006]    The current sensed at a receiving signal point is typically compared to a threshold value, and decisions about track occupancy, broken rails, and bits (e.g., codes, or signal aspects) are made based on this threshold. Since ballast leakage can vary with time and weather conditions, the threshold must be set to accommodate these changes while meeting the detection criteria for track occupancy (a short across the rails) and broken rails (an open break in a rail). A disadvantage is that this fixed threshold represents a joint optimization for detecting track occupancy, broken rails, and communication, but is typically not optimized for any one function. 
         [0007]    Existing approaches to jointless track circuits, used for example, in passenger rail systems, apply audio frequencies (@1 kHz to @10 kHz) voltages to the railroad track. The voltages are confined to a section of track by tuned shunts placed across the track at the block boundaries. The problem with this type of jointless track circuit is that the signaling points can be located only about 0.5 miles apart due to the low-pass filtering effect of the rail inductance. This type of circuit is not practical for rail applications requiring block lengths longer than 0.5 miles. 
         [0008]    A solution is needed that eliminates the insulated joints previously used to define a block of railroad track; that significantly extends the distance between signaling points; and that provides an inexpensive means for sensing track conditions. Additionally, to accommodate long distances between signaling points, it would be advantageous to place sensors along the track to help determine changes in the track model (e.g., to sense track conditions), or to act as communication repeaters. Such solutions will eliminate the maintenance costs and operational downtime associated with failed insulative joints. 
       BRIEF DESCRIPTION 
       [0009]    The present disclosure describes new methods and systems for extending track circuits and eliminating insulated joints that meet the needs identified above and provide solutions to the problems left unsolved by prior approaches. In particular, passive signaling devices (“PSDs”) are electrically connected to a railroad track. The PSDs are configured to place a programmable shunt impedance across the railroad track that can be used with voltages applied at the signaling points to aid in communication, train detection, and break detection for jointed and jointless track circuits. Signaling points can optimize the amplitude, modulation, coding, and frequency of waveforms that are applied to the railroad track (by signaling points) for at least three track circuit functions: detecting trains, detecting broken rails, and communicating between signaling points and PSDs. For example, train detection may require application of DC signals to detect a presence of train and AC signals to locate the position of the train. Alternatively, broken rail detection may require DC signals to detect breaks in the rails and AC signals to locate the position of the breaks. Additionally, communication of break detection and/or train detection data between PSDs and signaling points may require modulation techniques that have high spectral efficiency. Non-limiting examples of such modulation techniques include Pulse Amplitude Modulation (“PAM”), Quadrature Amplitude Moduation (“QAM”), Orthogonal Frequency Division Modulation (“OFDM”), and the like. 
         [0010]    A new passive signaling device (“PSD”) constructed according to the principles described in this disclosure has a unique operating sequence that can be used with signaling points to apply each of these different types of signals to the track in a duty cycle that is appropriate to the task. Thus, in some embodiments, train detection occurs frequently (meaning that the passive signaling device applies an AC signal to the track about once per second), whereas broken rail detection occurs less frequently (meaning that the passive signaling device applies a DC signal to the tracks about once per minute). In an embodiment, the PSD is a device placed between the track rails and powered through the rails by DC voltage supplied by a signaling point. 
         [0011]    Each PSD may include a switch (“PSD switch”). When the PSD switch is closed, the PSD can sense current provided by the signaling point through the rails. When the switch is open, the PSD can sense voltage across the rails applied by the signaling point. The PSD can communicate with neighboring signaling points or PSDs using the switch to modulate the voltage or the current provided by the signaling point. This is analogous to a passive RFID tag, which receives its power through the RF interrogation waveform sent by a reader, and modulates the interrogation waveform to send information back to the reader. Using this approach, low cost voltage and current sensing PSDs can be installed along the track (without needing to lay extra cables) and powered by a signaling point located miles away. Use of PSDs configured as described herein improves the communication range of data because each PSD can communicate data to its neighbors, which can relay the data back to the signaling point. The signaling point can then relay the data to the cab of a train or to a control point at the railroad. 
         [0012]    The PSD-based system and methods described herein leverage the fact that DC voltages (and low-frequency AC voltages) have the least attenuation in rails, and that an AC voltage/current can be generated on a rail by modulating the PSD switch when a signaling point applies a DC voltage to the rail. The AC voltage/current can be limited to a region on a rail by the rail inductance, and used to better resolve the location of rail breaks and the location of trains within a block of railroad track. More significantly, a PSD can be used to define a block boundary in place of an insulated joint. 
         [0013]    In an embodiment, a method comprises a step of feeding a DC voltage from a signaling point to a railroad track. The method further comprises a step of recording an amount of current received by a passive signaling device (“PSD”) that is electrically connected to the railroad track. The method further comprises a step of detecting a presence of one of a train and a break in the railroad track using the recorded amount of current received by the PSD. 
         [0014]    In another embodiment, a method comprises a step of receiving a data packet from a passive signaling device (“PSD”) that is electrically coupled to a railroad track. The method further comprises a step of processing a content of the data packet. The method further comprises a step of outputting as result of the processing an indication of one of NO BREAK, BREAK, NO TRAIN, and TRAIN. 
         [0015]    In another embodiment, a jointless track system, comprises a railroad track including a first rail and a second rail. The jointless track system further comprises a signaling point electrically connected to the railroad track. The jointless track system further comprises a passive signaling device (“PSD”) electrically connected to the railroad track at predetermined distance from the signaling point. 
         [0016]    In another embodiment, a passive signaling device (“PSD”) comprises a control device, and a current sensor coupled with the control device. The current sensor is configured to be coupled with a first rail of a railroad track. The PSD further includes a PSD switch coupled with the control device. The PSD switch is configured to couple with a second rail of the railroad track. 
         [0017]    Other features and advantages of the disclosure will become apparent by reference to the following description taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a more complete understanding of the new passive signaling device (“PSD”), the system and methods for extending track circuits and eliminating insulated joints, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0019]      FIG. 1  is a diagram of a PSD that may be constructed in accordance with the principles set forth in this disclosure; 
           [0020]      FIG. 2  is a system diagram illustrating how the PSD of  FIG. 1  may be configured and used to detect a train along a predetermined section of railroad track; 
           [0021]      FIG. 3  is a flowchart illustrating an exemplary method of detecting a train along a predetermined section of railroad track; 
           [0022]      FIG. 4  is a system diagram illustrating how the PSD of  FIG. 1  may be configured and used to detect a broken rail along a predetermined section of railroad track; 
           [0023]      FIG. 5  is a flowchart of an exemplary method for detecting a broken rail along a predetermined section of railroad track; 
           [0024]      FIG. 6  is a system diagram illustrating how the PSD of  FIG. 1  may be configured and used to communicate data to and from a signaling point; and 
           [0025]      FIG. 7  is a flowchart of an exemplary method for communicating data to and from a signaling point. 
       
    
    
       [0026]    Like reference characters designate identical or corresponding components throughout the several views. 
       DETAILED DESCRIPTION 
       [0027]      FIG. 1  is a diagram of a new passive signaling device (“PSD”)  100  configured configured to detect a presence of a train or a presence of a broken rail within a predetermined section (e.g., block) of railroad track (hereinafter “track”). The PSD  100  may also be configured to communicate track data to a signaling point. Track data includes, but is not limited to: data indicating a train is present within a predetermined block of track; data indicating a train is not present within the predetermined block of track; data indicating a train is approaching or receding from a PSD; data indicating a rail (or rails) within the predetermined block of track has a break; and data indicating there are no breaks with the rail (or rails) within the predetermined block of track. 
         [0028]    Referring to  FIG. 1 , a PSD may include a low-power control device  103 , a power supply 105 , a voltage surge protector  107 , a current sensor  109 , and a PSD switch  111 . The control device  103  may be any suitable type of device configured to operate the new PSD. Non-limiting examples of a control device  103  include: a microprocessor, a microcontroller, a programmable logic device, an oscillator (that periodically activates the PSD switch  111 ), and the like. The oscillator could be used, in an embodiment, to detect a break in “dark territory” over an extended length of railroad track. 
         [0029]    In an embodiment, the PSD switch  111  is a power MOSFET, and the power supply  105  is a DC-DC converter. Alternatively, the power supply  105  could operate from a rectified AC voltage supplied by a signaling point. The control device  103  may be configured to measure switch current and track voltage. Additionally, the control device  103  may comprise a processor, a memory, an analog-to-digital (“A/D”) converter, and analog and digital outputs. A non-limiting example of a suitable control device is one selected from the MSP430 family of ultra-low power microcontrollers manufactured by Texas Instruments of Dallas, Tex. 
         [0030]    Each of the power supply  105 , the voltage surge protector  107 , the current sensor  109 , and the PSD switch  111  couple with the control device  103 . The current sensor  109  connects to the PSD switch  111 . The current sensor  109  is configured to electrically connect to the rail  101  of a railroad track; and the PSD switch  111  is configured to electrically connect to another rail  102  of the same railroad track. In this manner, the PSD  100  is positioned between the rails  101 ,  102 , and may be buried in the ballast between them. Any suitable fastening means may be used to electrically connect the current sensor to the rail  101  and to electrically connect the PSD switch  111  to the rail  102 , as long as no complete breaks are made in either the rail  101  or the rail  102 . In an embodiment, a complete break is any type of gap that severs a rail  101  or  102  into two separate, electrically insulated pieces. Optionally, the electrical connections could be made through a low-pass filter to reject high frequency voltages that may be on the track from grade crossings or other track systems. 
         [0031]    Additionally, a V+ lead  115  may couple the control device  103  with the rail  101 , and a V− lead  117  may couple the control device  103  to the second rail  102  so the control device  103  can measure the voltage across the rails. Additionally, a positive current (I+) lead  119  and a negative current (I−) lead  120  may connect the current sensor  109  to the control device  103 , so the control device  103  can measure the current through the PSD switch  111 . 
         [0032]    In operation, V+ and V− provide inputs to an analog to digital (A/D) converter operated by the control device  103 , which processes the converted V+, V− inputs to monitor track voltage when the PSD switch  111  is open (e.g., off). Similarly, I+ and I− provide inputs to the analog to an digital (A/D) converter (not shown) operated by the control device  103 , which processes the converted I+, I− inputs to monitor track voltage when the PSD switch  111  is closed (e.g., on). The DC-DC boost converter steps up voltage that a distant signaling point sends through the rails  101 , 102 . The stepped-up voltage is used to operate the control device  103 . The voltage surge protector  107  protects the PSD  100  and its components from harmful electrical surges (caused by lightning strikes or other phenomena). 
         [0033]    The PSD  100  may further include a memory (not shown) coupled with the control device  103 . Computer-readable instructions may be stored within the memory that when processed by the control device  103  cause the control device  103  to perform one or more of the method steps described herein. 
         [0034]    In an embodiment, an on-resistance of the PSD switch  111  is between about 0.005 Ohms and about 0.020 Ohms, which is lower than the maximum shunt resistance specification of the train, so the total PSD switch resistance may be limited by quality of the connection to the rails. Current consumption to drive the PSD switch at about 5 kHz is estimated to be about 0.5 mA, of which about 0.2 mA is needed for the control device  103 . Total power consumption in one embodiment is about 1 mA×3.3 v=3 mW, which can easily supplied from DC voltage on the rail provided by a signaling point. 
         [0035]    Persons of ordinary skill in railroad signaling will appreciate that the exemplary configuration of the PSD  100  of  FIG. 1  assumes that voltage signaling on the rail is unipolar. Consequently, other configurations of the PSD  100  may be required for other types of voltage signaling. 
         [0036]      FIG. 2  is a diagram  200  illustrating how the PSD  100  of  FIG. 1  may be configured as part of a system and used to detect a presence of a train  201  (represented, for simplicity&#39;s sake, by a single axle and set of wheels) within a block of railroad track  203  that is defined between a first PSD  205  and a second PSD  206 . Additional blocks of railroad track  202 ,  204  are formed to the left/right of the block of railroad track  203 , respectively. It should be noted that  FIGS. 2 ,  4 , and  6  are not drawn to scale, and that the blocks of railroad track  202 ,  203 ,  204  may be any suitable length, but are preferably one or more miles long. Additionally, it should be noted that the PSDs  205 ,  206  are configured in the same (or like) manner as the PSD  100  of  FIG. 1 . 
         [0037]    Each block of railroad track  202 ,  203 ,  204  includes two spaced-apart parallel rails  207 ,  208 . The metal rails  207 ,  208  rest on a plurality of spaced apart railroad ties  209 , each of which is positioned orthogonal to the rails  207 ,  208 . Ballast  210 , such as gravel, occupies the spaces between the rails  207 ,  208  that are bounded on either side by the railroad ties  209 . The blocks of railroad track  202 ,  203 ,  204  may be formed between pairs of connections  211  that electrically connect the PSDs  205 , 206  to the rails  207 , 208 . 
         [0038]    A first signaling point  212  for communicating with the PSD  205  connects to each of the rails  207 ,  208 . A second signaling point  214  for communicating with the PSD  206  connects to each of the rails  207 ,  208 . In an embodiment, the PSDs  205 ,  206  are positioned between the points where the first signaling point  212  electrically connects to the rails  207 ,  208  and the points where the second signaling point  214  electrically connects to the rails  207 ,  208 . In use, the first signaling point  212  and the second signaling point  214  each provide current and voltage to the rails  207 ,  208 . The signaling point current and voltage are received and/or analyzed by the first PSD  205  and/or the second PSD  206 , as further described below. As shown in  FIG. 2 , a voltage pulse of about 200 ms duration may be applied. In other embodiments, different frequencies and different types of waveforms may be used. 
         [0039]      FIG. 3  is a flowchart of an exemplary method  300  for detecting a train  201  within a block of railroad track  203 , and is now described with respect to Table  1 . Table 1 is an example of a data structure that may be used to detect a presence of a train  201  within a block of railroad track  203  by comparing currents detected by a first PSD  205  and a second PSD  206  with predetermined combinations of current that represent different situations such as: No-Train, Train between a first signaling point (“SP112”) and PSD  205 , and Train between PSD  205  and PSD  206 . 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Train Detection Currents 
               
             
          
           
               
                   
                 Current @ 
                 Current @ 
                 Current @ 
               
               
                   
                 SP112 
                 PSD 205 
                 PSD 206 
               
               
                   
                   
               
             
          
           
               
                 No-Train 
                 LOW 
                 HIGH 
                 HIGH 
               
               
                 Train @ SP 1–PSD 1 
                 HIGH 
                 LOW 
                 LOW 
               
               
                 Train @ PSD 1–PSD 2 
                 HIGH 
                 HIGH 
                 LOW 
               
               
                   
               
             
          
         
       
     
         [0040]    Referring to  FIGS. 2 and 3 , the method  300  may begin at step  301  by feeding a DC voltage from the first signaling point  212 . At step  302 , the current from the first signaling point  212  is recorded. At step  303 , the current received from the first signaling point  212  by each PSD  205 ,  206  is recorded. The step  303  may include steps  307 ,  308 ,  309 , and  310 . At step  307 , one PSD within a block (illustratively PSD  205  in  FIG. 2 ) is closed. At step  308 , the current at the closed PSD is recorded. Then, at step  309 , the PSD is opened. At step  310 , this process may be repeated for the other PSD within range of the same signaling point (e.g., PSD  206  in  FIG. 2 ). Thereafter, the method  300  may proceed to the step  304  of detecting/outputting a presence of a train. Step  304  may include steps  311 ,  312 , and  313 . At step  311 , a data packet may be transmitted from both of the PSDs  205 ,  206  to the signaling point  212  or  214 . In an embodiment, the data packet transmitted by the PSD  205  contains the amount of current recorded when the PSD  205  was closed; and the data packet transmitted by the PSD  206  includes the amount of current recorded when the PSD  206  was closed. At step  312 , the currents detected and recorded at each of the closed PSDs  205 ,  206  are received the by signaling point  212 . A recorded current that exceeds a predetermined threshold is classified as “High.” A recorded current that meets or falls below the pre-determined threshold is classified as “Low.” After being received by the signaling point  212 , the recorded currents are compared to a data structure of the type shown in Table 1 to determine a train&#39;s presence within a block of railroad track (e.g., the position of the train  201  within bock  203  in  FIG. 2 ). If a train is detected, then at step  313 , either or both of the PSDs  205 ,  206  may be modulated at a predetermined frequency (or frequencies) to create an AC current to resolve the train&#39;s position within the block of track. Since a train approaching a PSD  205  or  206  creates an electrical short across the tracks, which changes the impedance (and thus the amount of current that flows through the rails  205 ,  206 ), the changes in impedance/current may be used in an embodiment of step  313  to calculate the distance the train is from either PSD  205  or PSD  206 . 
         [0041]      FIG. 4  is a diagram  400  illustrating how the PSD  100  of  FIG. 1  may be configured as part of a system and used to detect a broken rail  207  along a block of railroad track  203 . As shown, in  FIG. 4 , the rail  207  has a complete break  220  therethrough. The elements  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  212 , and  214  that comprise the diagram  400  are the same as those shown in  FIG. 2 , and for brevity&#39;s sake their descriptions are not repeated. 
         [0042]      FIG. 5  is a flowchart of an exemplary method  500  for detecting a break  220  within a block of railroad track  203 , and is now described with respect to Table 2. Table 2 is an example of a data structure that may be used to detect a presence of a break within a block of railroad track  203  by comparing currents detected by a first PSD  205  and a second PSD  206  with predetermined combinations of current that represent different situations such as: No Break, Break between a first signaling point (“SP112”) and PSD  205 , and Break between PSD  205  and PSD  206 . 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Break Detection Currents 
               
             
          
           
               
                   
                 Current @ 
                 Current @ 
                 Current @ 
               
               
                   
                 SP112 
                 PSD 205 
                 PSD 206 
               
               
                   
                   
               
             
          
           
               
                 No-Break 
                 LOW 
                 HIGH 
                 HIGH 
               
               
                 Break @ SP 1–PSD 1 
                 LOW 
                 LOW 
                 LOW 
               
               
                 Break @ PSD 1–PSD 2 
                 LOW 
                 HIGH 
                 LOW 
               
               
                   
               
             
          
         
       
     
         [0043]    Referring to  FIGS. 4 and 5 , the method  500  may begin at step  501  by feeding a DC voltage from a first signaling point  212 . At step  502 , the current from the first signaling point  212  is recorded. At step  503 , the current received from the first signaling point  212  by each PSD  205 ,  206  is recorded. The step  503  may include steps  507 ,  508 ,  509 , and  510 . At step  507 , one PSD within a block (illustratively PSD  205  in  FIG. 2 ) is closed. At step  508 , the current at the closed PSD is recorded. Then, at step  509 , the PSD is opened. At step  510 , this process may be repeated for the other PSD within range of the same signaling point (e.g., PSD  206  in  FIG. 2 ). 
         [0044]    Thereafter, the method  500  may proceed to the step  504  of detecting/outputting a presence of a break in either or both of the rails  207 ,  208 . Step  504  may include steps  511 ,  512 , and  513 . At step  511 , a data packet may be transmitted from both of the PSDs  205 ,  206  to the signaling point  212  or  214 . In an embodiment, the data packet transmitted by the PSD  205  contains the amount of current recorded when the PSD  205  was closed; and the data packet transmitted by the PSD  206  includes the amount of current recorded when the PSD  206  was closed. At step  512 , the currents detected and recorded at each of the closed PSDs  205 ,  206  are received the by signaling point  212 . A recorded current that exceeds a predetermined threshold is classified as “High.” A recorded current that meets or falls below the predetermined threshold is classified as “Low.” After being received by the signaling point  212 , the recorded currents are compared to a data structure of the type shown in Table 1 to determine a break&#39;s presence within a block of railroad track (e.g., the position of the break  220  within bock  203  in  FIG. 4 ). At step  513 , either or both of the PSDs  205 ,  206  may be modulated at a predetermined frequency (or frequencies) to create an AC current to resolve the break&#39;s position within the block of track. Thereafter, the method  500  may end. 
         [0045]      FIG. 6  is a diagram  600  illustrating how the PSD  205  (which corresponds to the PSD  100  of  FIG. 1 ) may be configured as part of a system and used to communicate data to and from signaling points  212 ,  214 , which are not in direct communication with each other due to signal loss along the track. The elements  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  212 , and  214  that comprise the diagram  600  are the same as those shown in  FIGS. 2 and 4 . For brevity&#39;s sake, their descriptions are not repeated. 
         [0046]      FIG. 7  is a flowchart of an exemplary method  700  for communicating data to and from signaling points  212 ,  214  and PSD  205 . Referring to  FIGS. 6 and 7 , the method  700  may begin at step  701  by sending a data packet from a signaling point  212  to a PSD  205 . The step  701  may include steps  705  and  706 . At step  705 , modulated voltage applied to the track from the signaling point  212  creates the data packet. At step  706 , the modulated current provided by the signaling point  212  is monitored at the PSD  205 . 
         [0047]    As the signaling point  212  sends the data packet to the PSD  205 , the method  700  may further include a step  702  of receiving the data packet at the PSD  205 . The step  702  may include step  707 . At step  707 , the PSD  205  receives the modulated current provided by the signaling point  212 . Thereafter, the method  700  may include a step  703  of sending a data packet from the PSD  205  to the signaling point  214 . The step  703  may include a step  708 . At step  708 , the PSD switch is modulated to create the data packet of step  703 . Thereafter, the method  700  may include a step  704  of receiving the PSD data packet at the signaling point  214 . Step  704  may further include a step  715  of applying a voltage to the rail and monitoring current modulated by the PSD  205 . In an embodiment, the voltage may be a DC voltage applied by a signaling point  214 . 
         [0048]    At step  709 , the content of the PSD data packet may be processed by a control device and/or compared with a data structure of the types shown in Tables  1  and  2  to determine one or more characteristics about a predetermined block of railroad track  202 ,  203 ,  204 . At step  710 , a result of processing the content of the data packet is outputted. The step  710  may include a step  711  of outputting a result of “NO BREAK,” meaning that a block of railroad track  202 ,  203 ,  204  has no breaks. Alternatively, the step  710  may include a step  712  of outputting a result of “BREAK,” meaning that a block of railroad track  202 ,  203 ,  204  has a break in one or both of its section of rails. The location (e.g., distance from a PSD  205  and/or a PSD  206 ) of the break within a block of railroad track  202 ,  203 ,  204  may also be specified. 
         [0049]    The step  710  may further include a step  713  of outputting a result of “NO TRAIN,” meaning that no train is present within a block of railroad track  202 ,  203 ,  204 . Alternatively, the step  710  may further include a step  714  of outputting a result of “TRAIN,” meaning that a train has been detected within a block of railroad track  202 ,  203 ,  204 . The location of the train (e.g., distance of the train from a PSD  205  and/or a PSD  206 ) may also be specified. After all results have been outputted, the method  700  may end. 
         [0050]    Attention is now directed to various embodiments of distances between PSDs and/or signaling points. Using PSDs between signaling points, the DC voltage from one signaling point does not have to reach to the next signaling point for the track circuit functions to work. This allows the distance between signaling points to be extended approximately 1.5×-2× further than the typical distance (e.g., @2.5 miles) that separates signaling points today. Consequently, using embodiments of the methods and system described herein, the distance between signaling points may be extended to about 5 miles. Increasing the DC driving voltage at the signaling points can extend this distance by about another 50%, to about 7 or 8 miles. The distance between PSDs is determined, inter alia, by the number of “blocks” desired between signaling points, and the resolution of the locations of rail breaks and trains within a “block.” 
         [0051]    Embodiments of the new jointless track circuit methods and system described herein are configured to co-exist with existing signaling systems. Consequently, signals to and from the PSDs are designed not to interfere with grade crossing and cab signals. 
         [0052]    Additionally, the PSD-to-rail interface (e.g., track circuit systems  200 ,  400 , and  600  in  FIGS. 2 ,  4 , and  6 , respectively) is configured so as not to cause significant loading to the grade crossing and cab signaling systems. This may require adding a low-pass filter between the PSD connection and the rail(s). Where AC signals are used to provide the jointless track circuit function, the circuits can be set up such that grade crossing frequencies are used to sense trains near the grade crossing, and such that other frequencies generated by the track circuit are used to detect trains away from the grade crossing. The track circuits are further configured so that they will not interfere with each other. For example, in one embodiment, spread spectrum signals are used to hide the jointless track circuit frequencies from the grade crossing equipment. Alternatively, each jointless track circuit (e.g., block of railroad track) is configured to operate at frequencies outside the shunt filters used for the grade crossing. 
         [0053]    The components and arrangements of the methods and systems for jointless track circuits, shown and described herein are illustrative only. Although only a few embodiments have been described in detail, those skilled in the art who review this disclosure will readily appreciate that substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing from the spirit of the embodiments as expressed in the appended claims. Accordingly, the scopes of the appended claims are intended to include all such substitutions, modifications, changes and omissions.