Power control for serious power was really hard until about 30 years ago, when power MOSFETs and IGBTs got good. There were a long series of not very good devices from 1900 on. Ward-Leonard drives, dynamotors, magnetic amplifiers, motors with multiple switchable windings, thyatrons, ignitrons, silicon-controlled rectifiers... People needed smooth control of high power, but it wasn't easy to get.

Now you can get IGBT modules able to handle the power for a locomotive motor. They're not that big - you can hold one in your hand. The next thing seems to be going back to MOSFETs, but in silicon carbide. Tesla switched from IGBTs to SIC MOSFETs. At last, high power control is a solved problem.