A modern electric locomotive optimum speed range is determined by the switching speed of their inverters, and while it is broad, it isn't quite as broad as you suggest. For the high-speed run by the Siemens Taurus they had to change the gear ratio between the traction motor and the final drive (Quill). A key component to making 3-phase asynchronous motors work in locomotive service is the ability to not just vary the frequency of the supplied power, but also the voltage. This is why the power inverters used in locomotives are called "4-Quadrant Variable Voltage Variable Frequency Inverters". As the switching speed of the inverters increases (to raise the frequency), the inverters ability to regulate the voltage decreases. The voltage is determined by the voltage of the input DC, but also by how long the GTO Thyristor, or IGBT is switched on.