Yes, the big change is in the architecture of the drive system driven by the availability of solid state power electronics technology.
Today whatever is fed to the engine is converted to something around 3kV DC for the link bus. Then the users of the power, namely the motor drives and hotel power are all fed from the link bus through appropriate solid state modules that create the right kind of AC with the right waveform and frequency. Almost no new passenger equipment is built with DC motors any more.
One thing that this architecture does is that it completely isolates the control circuitry from the nature of the high voltage feed. The only components affected are the primary HV transformer and the rectifier to feed the link bus, and also incidentally the feed from regeneration to the HV side, which again is usually through the link bus.
So the issue of line frequency then remains relevant only to the capability of the transformer and rectifier. Rectifiers generally are relatively insensitive to frequencies. Transformers have to have adequate sized cores to carry the power needed. Lower frequencies require larger cores, but a transformer capable of handling the necessary power at lower frequency will work fine with higher frequency.
Before the current architecture became feasible, the control system used tap changers on the secondary side of the transformer and fiddled around with power in AC form before feeding it through a rectifier to DC motors. So they were more exposed to line frequency, but still equipment like Arrow IIIs were built to handle both 25Hz and 60Hz a long time back, even though they had DC motors in their original incarnation.
Now the flip side is that any extra capability costs extra money, so one does not willy nilly add extra capability when you have no plan to use it. But merely adding the ability to handle 25Hz and 60Hz in effect just makes the transformer a little heavier, and costs slightly more. OTOH converting the existing 25Hz electrification to 60Hz is an enormously complex, quite expensive and time consuming process. Some day, when everything around New York becomes electrified at 12.5kV or 25kV 60Hz, it may be time to consider taking the leap, like they did in Mumbai with their 1.5kV DC system. But for now that effort is probably better expended in more pressingly needed projects.
