APPENDIX 7:  DCC REVERSING

At the moment a short circuit 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 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 another train. If this happens, the reversing unit gets confused since no matter what action it takes, short circuits 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, 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 causes no problem so long as one of the reversing gaps isn't crossed.

 Whenever possible, I place my reversing gaps at about the fowling points so that I can be assured that only one train can be crossing the gaps at any one time. Putting 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. If it matters, I use TTX's reversing module and recommend it.