"This question is not how many mainlines, but how many locomotives. Each pwr-56 can support up to 4 amps (56 watts at 14vdc). Allow a 25% safety, let’s use 3 amps. I base my layout on the estimate that most HO locos will not exceed .5 amps. So you can run 6 locos on a single power supply. Work it from there for your layout."

For the electrically challenged: how many locomotives can be run By a PWR-56 with two AR-1s and one AM-2S (only running 2 blinking crossing signals and the bell sound, for now) attached to the PWR-56?