Use a full wave bridge to feed the voltmeter.
Ebay has a bi-polar ammeter
DC Minus Zero Plus -5A-0+5A Analog 85C1 Analogue Ammeter AMP Panel meter

 

Mouser or Digi Key would be my first stop before Amazon.

 

not sure why it's necessary to measure voltage and current during normal use. voltage is most useful when measuring it at different points on the layout

you can measure the voltage without a bridge before the reversing switch. you could just put an led/resistor across the track to indicate there is power to the track.

not sure why a current meter is necessary except to indicate too many locomotives being used. the voltmeter would indicate a lower voltage

 

...
you can measure the voltage without a bridge before the reversing switch. you could just put an led/resistor across the track to indicate there is power to the track....

Well if you like to tinker johnfuller5, open up the power supply and connect the two meters in there. Put them in a seperate box like me

 

Thanks for all the help! Don't know why I didn't find them on ebay and total forgot about Mouser and Digikey... sadly Amazon has me trained.

gregc - it's probably just me. Like to put in troubleshooting tools when I'm doing stuff... plus my grandson will think the gauges are cool

 

Very nice!

 

while not using a bridge rectifier would mean swapping the leads every time you change direction ... [a voltage gauge is designed to read voltage in one direction only]..
if you use a bridge to feed the volt meter no leads would have to be changed, however there is a voltage loss in the rectifier of about 0.67 volt ..this just means that if the meter reads ten volts the actual voltage reaching the locomotive would be 10.67 volts or so ..
this small difference may or may not matter to you ...

 

However you have two diodes in use in the bridge for either current direction, which would be about 1.20V drop; less if using lowVf schottkys rather than silicon diodes. However the meter current draw is miniscule so the diodes Vf is probably smaller.

 

could be educational to see the effect of voltage on current and what happens to current when the loco is loaded or forced to stop.

no need for a bridge if both are connected before the reversing switch.

seems there are digital voltmeters (not multimeter) that would also show polarity that can be put after the reversing switch, again for education