Ever since the beginning of model railroading the user has desired more and more control over the operation of his model trains.  The first electric model railroad locomotives had only a single remote feature--the engine was either on or it was off.  Later variable power was added and the operator could then also control the speed of his model trains.  This was a definite advantage but still somewhat limiting since the operator could not back the train up.  The Lionel company first introduced a slide switch on the engine that could be thrown by hand that would reverse the engine and later introduced an innovative electrical-mechanical on-board (in the engine) motor control unit that would change the engine's direction by simply interrupting the applied AC track power.  This unit was later improved by adding a neutral state to allow the train to stand idle when track voltage was applied.  This motor control unit had a state sequence that moved from "forward" to "neutral" to "reverse" to "neutral" to "forward" etc.  each time the AC power was interrupted.  The motor control unit is often referred to as a "reverse unit" or "E unit". 
Today most model train engines are equipped with DC motors and the remote control of the trains direction is accomplished by applying either positive or negative DC power to the track.  However, when Lionel developed their reverse switch it was difficult to produce DC power and good DC motors were not available.  To this day the Lionel company uses AC power with the same basic electrical-mechanical motor control unit design. 
The Lionel company also introduced an innovative remote control concept for their on-board whistle.  Here a DC voltage of either polarity applied to the track would actuate a DC receiver in the engine that in turn connected track power to the whistle sound device allowing the operator to blow the whistle when the engine was standing idle or moving, all by remote control. 
No manufacturer of miniature AC powered trains has ever taken advantage of the fact that both polarities of DC voltage were available.  However, model railroad enthusiasts soon discovered that judicious placement of diodes in the engine would allow them to control two trains on the same track independently or to expand their remote control options to two on a single engine. 
Model railroaders that used DC track voltage did not have independent remote control options since the two DC polarities were already used to control train direction.  Only dependent options were available such as a reverse headlight coming on when the train was moving in reverse. 
Another capability that the model railroad enthusiast would like is a way to move engines independently on the same track so that multiple unit consists (more than one engine operating in a single train) could be constructed or taken down in the model railroad yard or a pusher engine brought up to aid a train stranded on a grade.  For the operator that uses DC track voltage, it is difficult to couple a moving engine to a stationary engine that occupies the same section of electrified track since both engines would respond to the same applied voltage.  For the operator that uses AC track voltage where each engine is equipped with a motor control unit, it is possible to couple a moving engine to a stationary engine since the stationary engine could be in a neutral state.  However, once coupled, the two engines would remain out of sync.  since each attempt to change the state of one motor control unit would also change the state of the other. 
Over the last ten to fifteen years a number of electronic control systems have been developed that attempt to solve one or both of the problems of independent train control and expanded remote control ability.  One approach is called command control and uses transmitter/receiver techniques to address single engines at a time.  With this technique, each engine is equipped with a receiver that is specifically coded or tuned to receive only one of a number of transmissions that are being sent down the track.  In this way each operator equipped with a transmitter can move his engine independently of other operators that have engines on the same section of electrified track.  One disadvantage of these control systems has been that only a small number of codes or train addresses are practical for a given system which means that additional engines over this number will be given codes that are already used. 
Some of these systems also have remote control options where an operator can turn on a bell or light on his engine independent of other operators.  Most of these command control systems have a means of controlling slaving engines together so that multiple unit consists can be made up. 
For the operator of AC train equipped with a motor control unit, the solution for multiple unit consists was a little less complicated.  In one instance an electronic motor control unit was developed that performed the same function as the original motor control unit but would reset to a specified state if the power was off for more then a specified period (in this case it was about twenty seconds).  This way if two or more engines were out of sync.  the entire consist could be shut down for eight seconds and once power was restored all engines would be in the same state. 
Although this motor control unit solved the problem of multiple unit operation it did nothing for expanding the remote control options. 
One application of this invention is to increase the number of remote control options of AC powered electric trains where the locomotives contain on-board reversing units.  The motor control units are generally sequenced through their states by interrupting the AC power.  The motor control unit used in Lionel trains have four distinct states: 1) forward 2) neutral before reverse 3) reverse and 4) neutral before forward.  For remote control, the Lionel company used a DC remote control voltage for their on-board whistle sound effect generator.  For this case the polarity did not matter since either a positive or negative DC voltage would turn on the whistle.  Although, this is a very limited electric train control system, it has been around for some time and most operators of Lionel trains are very comfortable with it.  They tend to resist using complicated digital remote control systems for a variety of reasons.  First they are used to using the simple power interrupt to change the direction of their locomotives--there is a quality of delightful simplicity in this.  They also do not want to alter their engines to the point where they would not work on other peoples train layout that use original Lionel transformers etc.  However, there is some dissatisfaction with the limited number of remote options that are available and some operators have used both positive and negative DC voltages to at least increase the number of remote control features to two. 
Adding the additional DC remote control voltage is about as complex a remote signaling system as most Lionel train operators want to go.  The challenge for a new AC remote control system is to increase the number of remote options and not change the basic simple and universal operation of the old Lionel control system that has been around for the last fifty years or so.