What does whether you have your feet on the floor matter?
If two people are moving forward against the direction of travel (one on the ground and one in the air) inside the train and the train suddenly stops dead (instant deceleration) both will move backwards at the same rate. They both had the speed of the train, for example 40 knots (regardless of their airspeed) and when the train stops they will for a moment retain that 40 knots and hence fly backwards towards the front of the train. The reason why they might not end up abeam each other is because the on the floor (having fallen over) will stop faster.