Contrary to seemingly popular belief, it’s not really for your safety that you don’t stand within that white box on the trains by the doors – notice that similar markings aren’t found on the high-floor Type 1 cars or by the non-bridgeplate doors in the 2s and 3s and 4s. The type 4 doors look a little different:
These are found only at the bridgeplate doors, and the reason you’re not supposed to stand there is that that part of the floor is a cover for the bridgeplate that lifts slightly to deploy and retract a bridgeplate. If several hundred pounds of people are on top of that cover when the motor inside tries to retract the bridgeplate, it gets stuck. Then the operator has to radio in to get permission to fix it, leave their cab, walk to the affected bridgeplate (which might be in their trailing car), fix the problem, and return to their cab. And now your train is late.
So, it’s easiest just to tell people not to stand on it!