Energy management in a crash often has more importance than mass. I have seen collisions between automobiles of different vintages where the older, and often heavier, car looks pretty good but the people inside took all the impact force and are a mess, while the lighter, modern car looks like a beer can after a hell's angels' rally, but the occupants got out without a scratch because the car took the impact, not the occupants. Mass can often work against you in some crashes.

While having a locomotive up front provides extra space, there are cab cars and EMU / DMUs out there with excellent crash protection.