The Milwaukee Road RRY had 665 miles of electrified line. The complete line ran from Chicago to Seattle, with portions served by 3,000 V DC lines, The later engines were called "Little Joes'  GE originally built the engines for sale to Russia and Josef Stalin. Cold war tensions caused the sale to be scrapped so the Milwaukee found themselves in a position to swing a decent deal on the engines. There are still a few of the brick substations standing along the abandoned line in MT. Those stations took 3-phase 110 KV AC and used motor generators to convert it to the 3 KV DC power that powered the engines. Of course, all that remains of the substations is the shell. Electrical components have long since been sold for scrap. 
Although the Milwaukee was a fair bit south of the Great Northern RRY, where I live, I did run into to several folks who worked for the RRY and picked up a few tidbits from them:
A good dispatcher could sync the trains so a locomotive going down a grade and using electric regen braking could have that power sent to another locomotive climbing a grade.
The lines to transmit the power were solid copper of a decent size. The catenary mechanism that connected the overheads to the locomotive were in constant contact and thus subject to wearing the conductors flat. One of the many reasons given for abandonment was the impending replacement of 665 miles of copper wires and the immense cost of doing so.