﻿ Trackwork Handbook
Trackwork Handbook

Appendix 7: DCC Reversing

At the moment a short circuit occurs at a reversing gap with mismatched polarity (e.g., caused by an engine crossing the gaps, a lighted passenger backing across the gaps, or even a metal wheel set contacting the rails on either side of one of the gaps), an auto reversing unit instantly switches the polarity of the reversing section so that the short circuit is eliminated. When all is working well, this happens so fast that the short circuit isn't apparent. So long as the polarity at the reversing gaps remains correct, as it would for the rest of a train crossing the gap, no further action is taken.

A reversing section, however, always has reversing gaps at each end of the reversing section. This means that the polarity is always mismatched at one end or the other of the reversing section. Trouble occurs when both gaps are being crossed at the same time, either by a train that is longer than the reversing section or by a second train. If this happens, the reversing unit gets confused because no matter what action the reverser takes, short circuits at one set of gaps or the other will occur that require it to take the opposite action. This will happen repetitively until at least one of the reversing gaps is clear of a train. Avoiding this problem requires either a track design that prevents the gaps at either end of a reversing section from being crossed at the same time or instruction to operators identifying the location of reversing gaps and establishing an iron-clad rule that only of the gaps can be crossed by a train at any one time (and, assuming metal wheelsets or car lighting, this means the *whole* train from engine to caboose or FRED).

One thing to be sure to check if a reversing unit doesn't trip is the track bus wire gauge.  If the feeder wire is too small (too high resistance), the reversing unit may not operate reliably.

Note that movement of trains within the reversing section has no affect as long as one of the reversing gaps isn't crossed.

Whenever possible, I place my reversing gaps at fouling points so that I can be assured that only one train can be crossing the gaps at any one time. Locating reversing gaps where two trains can cross them at the same time will cause no end of problems unless engineers can follow special operating rules perfectly. What I've done is completely trouble free. It's impossible for an engineer to confuse the reversing circuitry.

There is some discussion about whether the reversing gaps should be offset from each other. On Shandin, I located the reversing gaps directly across from each other and have observed no issues in the behavior of the reversing unit. You should check the most recent recomendations and locate your gaps accordingly.

Gary M. Green

gary@regalvalley.com