Polarity
If you are using an analog volt/ohm meter or an analog multimeter set the scale to 25 volts dc for a dc train power supply. Plug in the test leads red to positive, black to negative. Touch each terminal with the probes. If the meter moves forward and reads the voltage then the terminal you have touched with the red probe/lead is positive. If the meter moves backwards and does not read voltage then reverse the leads/probes at the power supply's terminals and the red probe is positive.
If you are using a digital meter simply set the the meter to read dc voltage. Touch the terminals with the probes. If the meter reads voltage without a minus sign by the number then the red probe is positive. If it reads voltage with a minus sign beside the number reverse the probes at the power supply to get the positive terminal.
