Automatic Phase Inversion

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Summary: Sometimes called "autoreversing", an auto reverser automatically corrects phase mismatches between track sections such as cross overs and reverse loops. Auto-reverse does not reverse the direction of a train, but inverts the phase relationship on one track segment.


Auto Reverse or Phase Inversion is an electronic feature found in some boosters, some of which are included in command stations. If the phase on the rails between two power districts controlled by different boosters do not match, a short will occur. To prevent that, both rails are gapped to isolate one power district from the other. When a metal wheel bridges the gap, a phase mismatch causes a short to occur. The auto-reverse circuit detects that event and flips (inverts) the phases so they match. Fleischmann/Roco use the term Auto Inversion which is more accurate in describing what happens.

Note: For neighboring power districts, only one booster should have auto-reverse enabled, so an endless loop of phase changes between two boosters doesn't happen.

Important: Digital Command Control does not have signals of different polarity on the rails. The direction of travel is determined by the instructions sent to the multifunction decoder within the locomotive.

Auto reversing devices are available, to implement automated phase matching on a reverse loop. They can be part of a power management device, or a stand-alone unit from suppliers such as Digitrax, DCC Specialties, Lenz, NCE and others.

If a train enters a reverse loop or balloon track, at the turnout's point rails Rail A will connect to Rail B through the turnouts wiring. To prevent this, there must be double gaps on the track loop.


The auto reverser will detect a phase mismatch across a gap in the rails, such as that found in a reverse loop or turntable, and instantly correct the situation. Any situation where Rail A connects to Rail B will result in a short circuit. The DCC signal rapidly switches from one rail to the other, while the other rail is held to ground until it switches rails again. This is why connecting Rail A to B results in a short. This also causes a short when two booster districts are out of phase.

Digital Command Control and Polarity

Digital Command Control has no concept of polarity. Despite what many will say, there are no negative voltages present on the rails, nor do the rails possess a specific polarity.

The correct term, as per the NMRA Digital Command Control standards, is Phase. The rails maintain a phase relationship, where one rail is always the opposite state of the other. Each rail has two distinct states, or phases: High (Energized/On) and Low (0/Off). When one rail is high, the other is low, and vice versa.

A short occurs when the two rails are bridged by a conductor, resulting in current flowing from High to Low.

The NMRA does define a rail as positive, yet that definition is subjective as it is determined by the direction the locomotive is facing.

Isn't Digital Command Control using Alternating Current


Alternating Current is an analog signal with a constantly changing amplitude and polarity, with respect to a ground reference. DCC is digital, it only has two logical states, On and Off.

Further Reading