Bachmann Reverse Loop controller
You need one of these automatic Reverse Loop controllers for DCC
when you have a WYE or a loop of track that comes back
on itself...visualize a single track with a loop at one end.
If you followed the right rail around the loop you would
see that it then BECOMES the left rail. Thus: A short
To avoid this you insulate the loop. Connect the Reverse loop
controller to the main track...and it's secondary to the insulated
loop. When a loco or lighted car hits the insulated section
it causes a minute 'short' that causes the controller to
reverse polarity in the loop to match that of the incoming
train...thus eliminating the operator having the need to
throw manual switches to do the same thing.