If you have a balloon or a wye, you will have a reversing section.  One way to effect the polarity change for these tracks is to use a relay attached to the switch machine at the throat of a balloon or wye.  aka a "throat controlled relay."  For more on reversing, see my website at:  https://wiringfordcc.com/intro2dcc.htm#reversing  Also, see my column in the April 2021 issue of Model Railroader.

Allan Gartner
Wiring for DCC
DCC Currents, Model Railroader

What is a "throat" controlled relay? I did a Google search of the net and could not find an answer. 

Thanks

Don