Finding the AC meters is always the tricky part, most of the digital meters are of the DC variety. A few years ago, I painted an old beat up PW boxcar and installed two AC 0-20V meters, one on each side. Obviously, I also had to install collector trucks to grab the voltage.
The tricky part was the meters required a totally isolated DC supply besides the measured voltage. So I had to cobble together a little supply that fed a 12VDC<->12VDC Isolated switching supply module. This powered the meters and they can then measure the track voltage.