There are hydraulic trains, they have poor running efficiency worse then electric. My understanding is in order to get a large train moving with steel on steel

There are hydraulic trains, they have poor running efficiency worse then electric. My understanding is in order to get a large train moving with steel on steel requires very precise traction control that lends it self to electric, while doing a mechanical coupling for cruise speed would be very complicated and expensive so they eat the efficiency loss there. They are also saving brake wear using the traction brakes which just run heat strips on the top of the train.