Not voltage, amps. Voltage is more like "speed", amps is kinda like torque.
Drawing more amps you may have a voltage drop, but too many amps trips a breaker. A 4a breaker can only pass 4a, be it 4a @ 12v or 4a @ 24v.
You can measure train, turnouts, and lamps alone and then add amps for total reading.
As you pass through turnouts, you may be firing two turnouts via anti-derail. (so e.g.. Turnout #1 uses 1amp, its lamp .25a #2 uses 1.2a, and .25a, Train one used 2.3a, caboose lamp .25a, Train #2 uses 3.5a.... 3.5+.25+.25+.25+1+1.2+2.3= 4.75a .... and mind you the engines might pull a couple of extra amps when first moving (split second; "peak amps" vs continuous) Thermal breakers usually trip on a continuous average of heat so peaks if fast are ignored; modern ones trip right at peaks which can be a pita if you are close to max amo draw all the time.
