The fastest lines on the old SR network are limited in repect of performance on DC:
a) none of them have linespeeds in excess of 100mph
b) the DC system itself limits the total current in a fed section to under 10,000A, i.e. 7.5MW - sometimes as low as 6,000A.
c) most EMUs had an average of one 250 or 275 HP EE507 traction motor per car, which in theory would allow over 4 x 12-car units freedom to run
Generally not a problem in the metro as power sectioning is for smaller areas so even when there are processions of trains running on yellows, their speed is low and they were coasting for much of the time. Even in the suburban area there isn't that much opportunity for trains to sustain top notch power for enough time to create overloads. Specific units like the class 432s (4-REPs) which were capable of drawing over 3,000A had restrictions on when more than one unit was coupled in order to not exceed the local capabilities of the DC supply. The class 422 which had half of the power of a class 432 per 5-car unit similarly draws over 3000A as a 10-car consist.
Roll time forward to the present where new design EMUs are designed with twice as much power per car:
class 444 - 400kW
class 387 - 420kW
class 450 - 500kW
class 395 - 560kW
The only practical fix for the DC power supply's inadequacies is to limit the maximum power draw. That is acceptable for the 387s and 395 as their main roles are working under OLE with effectively no power limitations. The SWR Desiros unfortunately operate crippled by their software on all duties.
3rd rail collection requires constant electrical continuity between the the conductor rail and the train's electrical circuit by a metal shoe. The shoe needs a significant pressure to ensure that the current isn't all passing through a high spot and not limited by rail head contamination. The higher the speed, the more pressure, however, a steel shoe on a steel rail would suffer significant wear if the pressure was high enough to give good pick-up at those speeds. So the compromise is that 100mph is recognised as the fastest practical operational speed. Higher speeds would result in the shoes bouncing over undulations causing continuous arcing and wear of the rail. This would be true even after profiling and geometry being addressed for higher speeds.
Although on DC, it is so slugged in its software that I expect it's performance isn't much better than an Electrostar.
The bottom line is that 3rd rail is essentially a metro system of power delivery. It has been stretched well beyond its optimum capability on 100mph track, but any daft ideas of higher than that is a waste of resources for ne real benefit.
