I think all posters here have made some good points. To summarize:
Watts are the way electrical (and motive) power is universally measured (which is the unit in question in the OP). .
It's also true that electrical current flow in Amps is the number of electrons flowing through a conductor in a given time time (6.242 × 1018 electrons per second) and this is what produces work (energy in the forms of magnetic fields and heat).
And without sufficient Voltage (Electromotive Force) to drive the load resistance, the work won't get done sufficiently to move the trains.
So then maybe the question could be restated to be: How many trains running does it take to equal the minimum load resistance (reactance) of around 3.27 Ohms before this circuit powered by a Z1000 will still function before its circuit breaker trips at around 5 to 5.5 Amps?
There are several different ways to figure out the answer to the question including testing by running 2 specific locomotives and some number of cars on a given track with the tightest curves and steepest grade on that layout and either measuring the current flow or making the Z1000 breaker trip.
