Are looking for just track voltage in general or voltage in a section as the train rolls down the track. If you want a freight car with a voltage meter in it, Lionel made a Great Northern work car with a meter some years ago .
If just track voltage in general, I use a digital multimeter from Harbor Freight with the probes stuck in to the ends of a spur track of Gargraves or tubular track. I have one attached to my transformer and one on a spur. That way I can see the voltage drop through the tracks.
At times Harbor Freight offers the meters free or for $1.00 with purchase . I bought a bunch of them, they last about 5 years at constant usage.
