DCC and Phase
Summary: While the DCC signal on the track does not have polarity, it does have a phase relationship.
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 reverser will maintain the correct phase relationships between a reversing loop and the approach track. The booster(s) will immediately disconnect 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.
Main article: Turnout
Often there are issues with older turnouts manufactured prior to the introduction of DCC. The most 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 both will be at the same potential. A wheel can bridge the gap between the stock and switch rail, causing a short. Checking wheel sets for proper gauging helps with this issue. Ensuring the stock rail and its adjacent switch rail are 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.