BART was always going to need custom trains for other reasons beyond track gauge. Electric third rail at those speeds isn't standard. 125kV pantagraph would mean big expensive tunnels and stations due to clearance requirements.
I also don't know where you're getting 125kV from. Many trains throughout the world use 25kV, especially high-speed ones (actually high speed, like 200+km/h), but BART uses 1000V, which is closer to a typical subway system.
