I was away for a couple of days. Jim, adding a capacitor across the outer rails would probably be a good idea. Part of the TMCC signal is conducted through the wheels to the frame and ground of the locomotive, and the capacitor would help with the reliability of that signal regarding dirty wheels and traction tires.
The RMT BEEP is a good example - only 4 wheels, and one of those has a traction tire. If the other wheel on the side with the traction tire is dirty, you don't have much to conduct the signal. If you run the BEEP without any cars, you may be in trouble.
There is also the possibility that your signal sensor circuit may already provide a path between the two rails at high frequencies, but that would depend upon your specific circuits.
The wheel sets of the cars will also help, but I have had some really dirty wheels that I wouldn't count on their conductivity.