There is actually an intermediate step between "friction brakes" and "regenerative braking" - electric trains can use the engines to brake, but the current prod

There is actually an intermediate step between "friction brakes" and "regenerative braking" - electric trains can use the engines to brake, but the current produced is then fed to resistors, which dissipate it as heat, rather than used to charge a battery or to feed it back into the power supply line - https://en.wikipedia.org/wiki/Dynamic_braking#Rheostatic_braking https://en.wikipedia.org/wiki/Dynamic_braking#Rheostatic_bra.... So the friction brakes are actually only used when maximum braking is required - and these will also be needed with regenerative braking. Using regenerative instead of rheostatic braking will however lower the amount of heat produced I guess...