I've been on both light rail and Amtrak trains when they went into emergency. Depends on how the system works, but as a practical matter if you're standing when it happens you will be on the floor unless you have something to hold on to or lean against. Depends to some extent how quickly the brakes come on. Amfleets & Horizons seem to come on very hard and quickly; Superliners and Cal Cars have a little softer start on the braking. With light rail, the old mostly-electromechanical Siemens cars stop Right Now in emergency, while the newer computerized cars seem to fade in and out a bit - though the maximum braking rate on light rail is considerably higher than on standard rail probably due to the magnetic track brakes that help out.

Leaves me wondering if HSR will need braking rates similar to highway vehicles (assuming that can be achieved with steel on steel) for emergencies. If so, they will need seat belts. Welcome to the airline safety spiel at every stop (better hope it's an express).