DCC Reversing Loop Question

[peahrens]

I recalled (triggered by another thread) that there can be an issue about train length in a reversing loop. That is, a lighted (passenger) train requires that the train length be slightly less than (fit within) the loop, otherwise when the loco hits the exit of the loop and triggers the loop "polarity" reversal, the rear of the train would bridge the then-reversed loop polarity with the opposite mainline polarity and create a problem.  The wiringfordcc.com site notes that the issue is just for lighted trains.  (See "Wiring Reversing Section").

http://wiringfordcc.com/track_2.htm#a19

 

My question is for other longer than loop trains with metal wheels, can these also cause a problem?  For instance, I put metal wheelsets on most of my freight cars, the axles being insulators between the two wheels.  Does it not create a problem if the lead loco reverses the loop when exiting, then (even at the same time) a caboose metal wheel hits the loop entry gaps?  I'm thinking the caboose wheel would cause a short at the entry gap(s) and cause the auto reverser to reverse again.  Maybe not a problem other than the reverser functioning multiple times, especially if the shorts occur loco wheel (at exit), caboose wheel, next loco wheel, next caboose wheel, etc.  And what about if a loco wheel hits the exit gap at the same time a caboose wheel hits a entry gap?  I may be overlooking something so would appreciate clarification.

[dehusman]

Short answer is yes, metal freight car wheels could.

The wheels are insulated between the rails (north rail to south rail), but when the rolling wheel crosses the gap its possible for the wheels or a metal truck to bridge the gap (north rail to north rail) and cause a short.

Installing a short insulated section (maybe an inch long) is one way around that.

 

[gmpullman]

The scenario you describe with lighted (or "lighting ready") cars has happened to me on a few occasions —when I'm not paying attention!

I have seen suggestions to slightly offset the gaps in each rail about 3/4" or so to help reduce some shorting but a metal wheel crossing the gap still has some potential to cause a short of enough duration to cause an unwanted reversal.

I believe some of the reverse devices have time and/or sensitivity settings that can be used to reduce the chances of a false short.

Ed

[MisterBeasley]

I use insulated rail joiners for this purpose.  The Atlas ones have a blocking piece of plastic to keep the rails from butting together and causing a short.  This is slightly higher than the rail head, and lifts the wheels a fraction so they don't end up in contact with both rails simultaneously.  I don't know if this is by design or just luck.

I use Tony's Trains auto-reversers.  They are solid state and flip polarity very quickly.  When I do run those lighted passenger cars, I see the auto-reverser indicator light flashing continuously, but the train seldom stalls or shorts.

I have a lot of older rolling stock left over from my days as a teenage modeler back in the 50s and 60s.  Many of these cars had metal trucks with plastic wheels.  I've replaced all the plastic wheels, but I still have a few metal trucks in service.  Those would have the same issues as "lighted" cars.