Why are special provisions required for a reversing loop situation?

greenspun.com : LUSENET : Wiring for DCC : One Thread

Hello All,

From what I have been able to gather regarding DCC, 2 trains, on a single block, can travel in opposite directions. This would lead me to believe that a reversing loop would not require any special wiring. (Admitedly my electrical knowledg is limited) As I have been looking at DCC, I notice that it requires special provisions when a reversing loop/wye, etc are involved, which appear to be no different from those in other engine powering layouts. The one thing I haven't been able to find is, "Why?"

I guess, related, are there engine units for DCC that make it so we would not need this special wiring?

My apologies if this has been answered elsewhere and I simply missed the answer.

Karl S

-- Karl Sumwalt (ksumwalt@kci.com), February 23, 2005



I think a picture says it best. Go to my webpage and look at figure "C" of the wyes. All wyes and balloon tracks have the same problem, but figure "C" shows it best.


Look at the top horizontal track. See how it is red on one side and directly across it is black? And just the opposite on the rail just below it? If these tracks were adjacent to each other without the blue/green reversing section, there would be a short circuit when the locomotive crossed over the gap.

The blue/green track first matches the polarity when the train enters the blue/green section. While the train is in the blue/green section, the polarity reverses to match the other side. The train travels through without a problem!

Note: a reversing section has nothing to do with a train going forward or reverse.

-- Allan Gartner (wire4dcc_admin@comcast.net), February 23, 2005.

Allan is correct in his diagrams. The reversing track problem is peculiar to running trains on 2 electified rails and is not much different with DCC than it was with DC throttles. Anytime the track reverses itself such that the right rail bumps into the left rail (such as in a ballon lop), there will be an electrical short circuit. This has to do with the electrical polarity of the rails with respect to each other rather than the actual direction of travel. But with DCC we have a way to handle it automatically without having to flip all those toggle switches. However, sometimes reversing tracks are not that easy to find.

One easy way to figure this out for yourself on any layout plan is to imagine that you are the engineer driving the train. Start anywhere and proceed down the track. Mark the right side of the track with an R. As you pass each track switch (turnout) mark again what you see as the right side of the track. As you pass through the layout take every path. If you ever encounter a section of track that you have taken before and find an R on the wrong side, you just found one end of a reversing track section. DonV

-- Don Vollrath (dvollrath@magnetek.com), February 25, 2005.

Moderation questions? read the FAQ