Summary: While the DCC signal on the track does not have polarity, it does have a phase relationship. Phase describes the relationship between the rails of the Digital Command Control signal.
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. 
The Positive Rail
The NMRA/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.
Phasing is very important. When wiring for reverse loops, power or booster districts it is important that the correct phase relationships be maintained.
For a reverse loop a phase inversion device can automatically correct any phase mismatches between the mainline and the loop.
For other applications, the wiring must be done in such a manner that the correct relationships are maintained. To determine if there is a phase mismatch, measure the voltage across the gap between two districts. If your meter indicates a difference in potential by displaying track voltage, the phases are out of sync between those two segments. If the meter displays little to no voltage, both segments are in phase.
Determine where the issue lies and correct it. A booster district mismatch can be fixed by reversing the connections for Rail A and B at the booster. For power districts, connections at the power management device will require correction.
- Main article: Reverse sections
Although there is no electrical polarity, you still have to deal with reverse sections on your layout. The phase of the rails is important in DCC. Rail A is aways the inverse of Rail B. If the track turns around back onto itself, the right rail (A) will come in contact with the left rail (B), creating a short circuit by connecting one phase to the other; the same as placing a metal object across the rails.
If the rails are out of phase across a gap, there is a potential difference between them, which can be measured. Should a metal wheel bridge that gap, current can flow from the energized section into the unenergized section, causing a short circuit. An auto reverse will maintain the correct phase relationships between a reversing loop and the approach track. As the direction of travel is determined by the decoder, the train will continue to move as if nothing happened. If an auto reverser is not used, the booster(s) will immediately disconnect during the short to avoid damage to their outputs caused by excessive current flows.
The same issue can happen if the frog in a turnout is incorrectly phased, the point rails are electrically one unit and not isolated, or with point rails which are insulated from each other, a wheel lacking the the 3º taper bridges those rails at the heel of the frog.
Matching phase 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 occur as the two boosters attempt to maintain the correct phase relationships.
- Main article: Turnout
Often there are issues with turnouts manufactured prior to the introduction of DCC. The likely culprit is the gap between the stock and switch rails. When the switch rails are wired together, as often found in older designs, they will both be at the same potential. A metal wheel can bridge the gap between the stock and switch rail, causing a short. Checking wheelsets for proper gauging helps with this issue. Ensuring the stock rail and its adjacent switch rail is in phase by modifying the turnout is another option.
The phase relationship between stock and switch rail is the main feature of the DCC Compatible turnout. It is also important that the point rails are not connected together, and if they are, that each one has a gap between it and the downstream trackage.
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.
- ↑ Many modellers will make statements referencing a negative voltage on the rails. This is based on observation of an oscilloscope's trace of the DCC signal on the track. It is misleading, as the track is floating there is no zero reference.
- ↑ There are no negative voltages, but the current flows from High to Low. Thus, the direction of current follows which rail is high at that point in time.
- ↑ Sometimes referred to as the points, or inaccurately labelled as point rails.
- ↑ The Point Rails are a part of the frog, which come to a point at the toe of the frog. Confusion arises from inaccurate terminology, confusing the switch rails with the point rails.