Some brands use the metal frame as the return wire. That makes their wiring easier. It makes converting to 2 rail different. I'm not sure of the MTH model you list. Some have plastic bottoms. Either way, the two wires are the same in theory. You also have to look at the new trucks to make sure both sides are insulated. I believe these are.
Sometimes you have to look at the whole wiring scheme to make sure your changes are correct. Other than that, it is very simple wiring. I've had it where I've had to use plastic coupler boxes to keep the cars from shorting out to each other, when turned around on the rails. That's when (on metal cars) the trucks have only one side insulated. I actually got shocked once, touching a set of Lionel metal hoppers that had NWSL wheels with only one side insulated. The originally had metal KD couplers and boxes that I installed. I had to go back and swap those out.
