I'll bite. There are two primary methods I've used:
1) Power limited. Consider your losses. Even the hyperloop project will have some losses (going up hills?). Every motor will have a continuous rating, which is what you should size for. At some point, as power(vel), you will reach the max power that can be output by a given motor.
2) Gearing limited. Every motor has a max RPM. You need to decide what type of transmission you can use, then your torque needs. Torque and max RPM are inversely related if you only have a single ratio. This assumes that you have an acceleration requirement, and know the mass of your vehicle. It's also important to know all your losses here, because a very low acceleration requirement may be dominated by the actual losses.
If your losses are truly negligeable and acceleration is the most significant, then your max velocity may alternately come from safety requirements or time-to-accelerate between stops.
I'm currently building software to do these calculations, and have quite a bit of analysis code producing visualizations already.
PM me if you're interested in engaging a consultant.