For most DC traction motors, you are probably safe up to 5,000 RPM (if you can get them to that speed - some tend to self-limit to around 4,000 RPM). Some motors built for other applications are only meant for low speeds - say 2,000 RPM or so and may not be strong enough to handle the centrifugal forces from higher RPM.
If the motor is still in production, you may be able to find specs online. The DC series motor I have seems to be OK with continual 4,000 RPM, and I have had it to about 6,000 once (estimated).
Some AC traction motors can spin near 10,000 RPM, but check the specs for the particular motor.
Here is a link to a few motors that were 'oversped'...
Blown Commutators