Bachmann F7A is the power for all passenger service . This 4 wheel pickup / 4 wheel drive pancake motor model is plentiful and cheap. Most modelers detest the pancake motor but I find them easy to use and reliable as long as you can get power into them uninterrupted, and they leave plenty of room for other mods.
AHM GP-18 is the power for most freight service. This 4 wheel pickup / 8 wheel drive GP18 can be operated either-end forward, so we don't need to turn the loco.