Just to give some dimensions to the current draw of Postwar Flyer locomotives with Universal AC/DC motors, attached are links to the factory service manuals with upper specification limits on the current draw under various loads.
http://myflyertrains.org/gallery/album209/285_295_1
http://myflyertrains.org/gallery/album209/4745_1
You can see that a typical AF steam locomotive with smoke/choo choo may draw a maximum of 2.1 amps at 12 volts when pulling four boxcars. An dual-motored Alco AA set would draw up to 3.25 amps at 12 volts while pulling four passenger cars. Although the Electrical Engineers among us may quibble with applying this relationship to AC, I have always used the approximation of Volts x Amps = Watts. Therefore, to be able to run your dual-motored Alco up to wide open throttle (15-16V on most Flyer transformers), would require a transformer of at least 52 Watts (16V x 3.25 amps). And that does not include capacity for anything else such as switches or other operating accessories or even lighted cars.
This explains why Gilbert typically included a 4B 100 Watt transformer with their dual-motored diesel sets.
