Well, you don't need belt drive for that. My VL-BB pulled 70 cars and seemed it could handle a lot more if I was so inclined. The is a point of diminishing returns with the gear ratio. Something around 26-28 to 1 would be about right. If the gear ratio gets much higher than that, you start having a whole new set of issues.
I did a couple of Williams brass steamers with 44:1 gearing. Once you got past around 30 scale MPH, the motor was starting to really create some noise. The scale speed topped out around 44-45 scale MPH at 8,000 RPM of the motor, and you could REALLY hear the motor. I will allow for most running, the 30 scale MPH would probably be fine for me, but it's just the idea of being "limited". 
