There are some "industrial" uses of series too, I noticed the steam rollers while driving through a construction area had hydraulic hoses to the "wheels".
IIRC hydraulic is more efficient than electric at regen than electrical. An add-on hydraulic brake energy recapturing system could do wonders.
Everything is huge on a train, plus stop and go, ya friction pads would be put to the test. An electrical coupling is probably less maintenance, I don't know if they did a comparison w/a gearbox efficiency in making the decision though, but the electric driveline is good for many many miles and lots of load
They also do rolling resistance (and streamlining) right on trains
So we have to be careful w/efficiency comparisons there also, including economies of scale, which favors lower BSFC.