On most trains the brake cylinder pressure (which determines the braking power and therefore deceleration rate) is the same for full service and emergency. The only difference is that emergency will sometimes be unreleasable until stopped, might apply faster, if fitted will often activate sanding equipment, and might disengage wheel slide prevention (as it can increase stopping distance compared to just allowing the wheels to lock, and having to reprofile the wheels because they have flatted is cheaper than rebuilding the front of the train because it's hit something). On Electopnumatically braked trains, emergency is almost entirely failsafe too, wheras the other steps might either not be failsafe, or far more at risk of wrongside failure.
