I thought the problem with Southern was that the speed limitation is the amount of power you can get into a system running at only 700V or so. The DC-AC conversion is much more efficient than the old brute force motor controller gear, but even so the capacity is determined by a formula which has to take into account number of trains on the line, operating speed, stop and start time and thermal effects.
The dirty great rail has to be connected by cable to the power station, and that's another limiting factor.
I used to enjoy explaining to visitors why the power going into a plating bath was no more than that going into a washing machine, but one of them had 300 square mm cables bolted to substantial busbars and a rack cabinet full of transformer and rectifiers, and the other just had a 3 pin plug an an ordinary mains lead.
Biting the hand that feeds IT
