Summary: Phase describes the relationship between the rails of the Digital Command Control signal.
In Analog, two conditions determine the direction of the locomotive: Amplitude and Polarity. The amplitude of the voltage determines speed, and the polarity or direction of current flow controls the direction of movement. Changing those parameters directly affects the speed or direction of your train.
Phase in Digital Command Control
The term Phase is the preferred term to describe the relationship of the voltages between the rails.
In Digital Command Control there is no concept of polarity. The decoder controls both speed and direction. The PHASE of the signal on the rails is important, as incorrect wiring will cause a short circuit. No negative voltages are present on the rails, they are either logically high or low. DCC does not use a negative voltage on the rails. As shown on the oscilloscope traces the rails alternate between On and Off.
Phase Sequence is important. In a reverse loop Rail A eventually connects to Rail B and both rails must maintain the correct phase relationships. In this situation a short circuit results, as one rail is energized and the other is at "0" potential. To prevent this, the loop must be isolated via gaps in the rails and an autoreverse device is used to maintain the phase relationship as the train returns to the mainline. The autoreverser can reverse (invert) the phase relationships seamlessly when a phase mismatch occurs during entry to or exit from the loop. As direction of travel is determined by the decoder, the train will continue to move as if nothing happened.
Unlike analog, where once the motive power is in the loop, the polarity of the main line is reversed to avoid a short circuit upon exiting the loop. The loop itself also needs to have the capability of polarity reversal to match the direction of the train entering it. Remember, with analog you control the track, not the train.
This is very important with booster districts: Both boosters must be in phase to ensure trouble free operation as trains move across the gap from one district to the next. Also, only one booster should have the autoreverse mode active otherwise an endless series of phase reversals will happen as two boosters attempt to maintain the relationship.
The Positive Rail
The NMRA Standards refer to one rail as being positive, which is determined by which direction the locomotive is facing. This term (positive) is strictly to establish a reference point, it does not indicate polarity.