Thanks Jerry. You're right. The old standby, worm gear and bull gear provide lots of reduction and very quiet too. Ray, the wheels are just LGB, off some four wheel euro passenger coaches.
In the end, I went with an Aristo/Delton powerbrick, for reliablility, pulling power, and silky smoothness.
Not trying to discourage anyone from tinkering with sprockets and chain. It can be lots of fun.
Not to hi-jack this thread, but in the experiments leading up the railtruck I also tried the Tamiya gearbox kit...
...and servo drive
I've heard good things about the Hartland unit in the rocket car.