You might want to consider Del Tapparo's "Critter control". It is not radio control but can be controled by track magnets. I bought three of them to control my RGS Geese but have not had a chance to install them. He also has one which senses other equipment running on the track and modifies its speed accordingly, really cool to watch.
Jim - My next product release will be a "Simple Critter Control" which you may be interested in. It is a motor controller with a built-in speed potentiometer. All it does is manually control speed, nothing else. It is designed specifically for roof mounting. The speed pot protrudes through the roof (can be disguised as a vent or smoke stack), and the board, which is only 1 inch square, is flush with the roof hanging down by only 0.7". This will be the smallest and lowest cost solution I offer.
Here is a sneak preview. From left to right are the Simple, Basic, and Enhanced Critter Controls. You can see that the Simple Critter Control will be quite compact. I expect to be able to ship the "Simple" boards in August.
This should be a nice solution for hiding controls in open cars such as Jim's trolley. My motivation was my Hartland Woody. This control can easily be roof mounted or concealed in the small weight box under the chassis.