From the Lionel Engineering Standards, in the early post war years they used #24 solid push back wire. It is rubber insulated with a fabric jacket. With push back insulation the rubber is not adhered to the conductor. The wire is cut to length, but not stripped. The insulation is just pushed back up the wire, the solder connection is made, and then the insulation is pulled back down the wire up to the joint.
The smoke units used #22 stranded wire with 7 X 30 stranding. The insulation was Vargas silicone insulation. I am assuming that this is a fiber glass fabric with a silicone resin binder.
When they moved to vinyl insulated wire, the conductor was still #24. The stranding was 42 X 40 or 7 X 32.
Recently someone has made the rubber insulated, fabric covered push back wire available for toy trains. Many of the parts and light bulb dealers have it. It is very easy to work with and comes in red, green, yellow and black. There is also a super flex fabric covered wire. Many of the original fabric covered wires were color coded, but the carbon in the rubber has stained it all black. Some times the color can be seen at the end of the wire where the fabric was loose and not in contact with the rubber. The color codeing is normally yellow for connections to the brush holders, green for field connections, and power was black. The use of red appears to be inconsistent. When two motor locomotives were introduced and the brush holder connections had to be the correct orientation to get the two motors to run the same direction, one of the brush holder wires was changed to blue. But this occurred after the insulation was changed to vinyl, except for the earliest F3s.