If this is the original postwar 342 then this should be pretty easy. Run one wire from the "A" terminal of the LW to one terminal on the 342. Run another wire form either the "C" terminal (14 volt) or "B" terminal (18 volt) to one side of a SPDT switch like a Lionel 90 or 364C accessory controller. Lastly run another wire from the other side of the switch to the other pole on the 342. I'd start with the A-C connection and then if that does work well enough try A-B.
Hope this helps,
Kevin
