M1 is powered with 2 Northwest Shortline powered axles installed in the same truck. It's fairly prototypical. Most motor cars powered only the truck at the end where the engine and controls were located. These were usually mechanical linkages, gears or chains, so powering the second truck was a much bigger engineering project.
One of the design considerations was maintenance. The copper wires showing in the middle of the underside of the truck secure the tab on each powered axle, preventing it from rotating. I can undo the wires, and fold the axles apart to get to the wiring, if I need to disassemble the truck.
The open geared axles aren't really designed to work outside. The grease on the gears soon picks up enough crud to jam, and the whole show stops. I made a gear box cover from 3 x 5 index card stock, stiffened by soaking in ACC. It comes apart in two halves, split on the axle, and is tied together with thin copper wires. There is a cover for each axle gear box. The axles have to be folded out, in order to get the gear box cover halves off for cleaning and lubrication. This system has worked very well. This car has run for several years. Sometimes for 6 hours a day. I have only had to lubricate it a couple of times.
There is one important modification to make before installation. The largest gear is only pressed onto the shaft that holds the wheel. If the gear heats up a little, it can slip on the shaft, with loss of power to the wheel. Take the axle completely apart, and pin the gear to the wheel with a small piece of brass wire. I drilled into the insulator bushing on the wheel. It doesn't have to be much, just enough to key the gear to the wheel and shaft.
I used two axles because I wasn't sure how much power I would need. I'm pleased with the result, but I imagine that a single axle would do for a car with no trailer. I like the idea of pulling a trailer coach or two. The car by itself has pulled up a 20% grade with the two axles.
I originally planned to run this car on a Makita 9.6 volt nicad battery. That is the system I use for my 4-6-0's and it's easy to change batteries. However, even the fastest gearing from NWSL was WAY too slow. I changed to two 6 volt gell. Even at the higher voltage, maximum speed is about 25 mph, which is acceptable. The NWSL literature warns that running the motor over 18 volts will shorten it's life. These are nominally 12 volt motors, so you want to wire them in parallel, for best operation.