The 474/475 PA/PA Rocket set has a dual motor powered 474 PA and an unpowered 475 unit with a diesel horn in it. A few 474’s were made in 1955 with a single motor but they have a diesel roar unit in the engine as well, so that will have wiring for powering the roar sound. If the chassis has one motor and no diesel roar unit it appears something was changed by a past owner.
Pardon my sloppy sketch, here is a diagram of what the reverse unit is doing. The bulb is wired across the two pickups no matter the position of the reverse unit. The armature and the field are in series. In one position the connections as shown in red are made, in the other position the connections as shown in dotted blue are made. The reverse unit is a four position design so it cycles F-N-R-N. The field connections are not completed when the reverse unit is in the N positions. Regrettably I do not have one of those diesel reverse units handy to show a picture of which wires go to which finger in that specific reverse unit.