I would like to add that, for some of the reasons in my post above, my personal favorites are actually the Pittman 8000s. I've tested 8000s, 9000s, Mabuchi's etc. I've yet to find a traditional DC brush motor that's really content and consistent below 800 RPM in a model train application.
To get full points in an NMRA contest, a loco has to run at 4 MPH. (That also happens to be a realistic coupling speed.) So you want the loco to run consistently, 800 RPM = 4 MPH.
A Pittman 9000 only revs to about 6400 RPM. This is 8x the minimum speed, giving a maximum of 8 x 4 = 32 MPH! The 3rd Rail Decapod (1990 model) used this formula with great success. It was one of the smoothest runners before the era of speed control. But most would want their road engines to run faster than that.
So we substitute an 8000-series Pittman with a redline of ~9,600 RPM. Now the top speed is 12x the minimum speed, or 48 MPH. If you're willing to leave a little bit of slow-speed performance on the table, you could gear it such that 800 RPM = 5 MPH with a top speed around 60+. For a steam loco with 68" drivers, the gear ratio would be 32:1. This is probably the ideal combination for most of us with bedroom-sized layouts.
My hypothesis is that the 9000-series saw more use early in the Scale era because most locos had super-tall "toy train" gear ratios. If they had been geared like scale models in the first place, a higher-RPM motor would have been specified, and you would have better performance even without speed control. Of course then the manufacturer would have to incorporate provisions to tame NVH at high RPMs (such as mounting the motor with rubber grommets), and it would have cost a little more. But it would have been a better product. The perfect loco hasn't been made yet in O gauge and my wallet is still full, hmm...
