I took an old Lionel MPC junker boxcar and tacked a few blocks of 2"x2" steel to its frame, replaced the plastitrucks with good sprung trucks and stuffed a kinda alcohol soaked scotchbrite sponge underneath. The sponge was too tall to fit, the steel kept it tight to the track and the trucks soaked up the discrepancies in my track laying abilities.
Follow that up with a cloth under another car and it worked just fine.