Dominic, we're lucky to have SnS Trains in Idaho which make a "bolt in" motor conversion available for old Flyer trains. I seem to recall from when I was doing O gauge three rail several years ago that there was an outfit on the Internet which did something similar for Lionel post-war trains which included a small flywheel for smoother starts and stops, but I can't recall the name.
One thing I have found that helps smooth out operation is to use a modern power pack. The old transformers are part of the problem because they are not sensitive enough to start a train at lower voltages. The MRC pack for use with G scale trains is powerful enough for heavy old toy trains. I've been using the MRC 6200 for many years, but it has now been replaced by a better pack.