That is the official way to stop a train and is taught by everybody who will be lineside that that is how you stop a train in an emergency.
It is set down by the HSE that you need a Hand Signal for stop at the bare minimum to operate a railway if there is a situation that arises where a train needs to be stopped.
I can guarantee, if you wear a Hi-Vis vest, and you go to the end of a Platform with a train approaching that is due to go non-stop through, and you do that Hand Signal, it will stop as fast as it can.
Oh and if you do that you'll be arrested