BPCS button is present in 3 phase locos( P7 is also a 3 phase loco) which activates Autopilot mode. When Autopilot is activated Tractive effort is controlled by micro processor and in that case Overspeeding upto +/- 5 km/hrs can happen. This is about 3-phase locos, now talking about conventional locos.They generally overspeed when the train is descending a gradient, Voltage variations and here it would have a magnitude more than 3-phase locos. Overspeeding happens dear and no one can deny that. Aaj kal logo ko lagta hai ki jo unke Muh se nikal gaya wo bramhaya wakya hai or unki soch se viprit baate bolne pe unhe bura lagta hai isliye hum bhi faltu ka gyan nahi baatne ki kosis karte hai per kabhi sacchai nikal jaati hai