This is not a required item! You don't need them unless you have one of the problems listed. To prevent problems on a new layout follow the recommended wiring guidelines. Alan Gartner’s wiring for DCC - http://wiringfordcc.com/
THAT ARE THE SIGNS YOU MAY NEED A RC FILTER ON THE TRACK BUS?
Do you experience the following?
1) Loss of train control. 2) Runaway engines. 3) Loss of decoder programming. 4) Decoders blowing up.
WHERE DO I INSTALL THE RC FILTERS?
Put the very first one on a given track bus at the FAR end of long track bus run. Repeat for every track bus you have. Why at the FAR end? The booster already has some RC filtering capability built in which covers the starting point of the track bus. The RC filter you install will provide the same capability at the opposite FAR end. Together they work on filtering the middle of the track bus. A divide and conquer approach to the problem that has its roots in engineering principles.