Not all milk cars had a dashpot to regulate the action. Voltage is the way to go on those.
While the jams are a pita, it may also be a vehicle to try and teach some patience with. Your explaining a small pause will help it function more regularly by giving the next barrel time to drop shouldn't be too hard to grasp if they choose too 
Besides that, they all seem to jam sometimes. I'm guessing my favorite dry lube, T-9 by Boeing, on the ramp walls would help a lot. (teflon based spray dries to a very thin wax-like coating, dry but very slick. Sparse use is all it takes and it lasts too)
It used to be a bit pricey, but you really don't need much at all when used. Sticking to just the hobby a can just may be a lifetime supply. You'll find other uses too, especially were oil/grease or graphite tend to be a bit if a mess.
