How about drilling 2 small holes and using some short dowels(or maybe rods with electrical tape around them to protect from scratching up the bumper on the trolley too much)?
Insert dowels for bump n go, remove for normal use. The holes in the track bed probably wouldn't be too noticeable.
These would have to be slightly far away from the boxcar if you don't want the trolley to actually hit the coupler on the boxcar. Not sure if that ruins the illusion for you or not. (Though I see your drawing seems to be missing the coupler on that end anyway, so maybe a missing coupler is part of the plan?)
Then again, if you want, it may be possible to do something similar but oriented so as to keep the truck on the boxcar stationary (like the dowels under the truck, maybe right up against an axle), but that might be a bit of a risk to the boxcar if you kept hitting the coupler with the bumper of a bump n go or #50..
-Dave