Confused with HST and 225 from WSML ECML

As much as this is a stupid question when I'm driving the HST and 225's that I got with the east coast route and west coast route, theykeep on slowing down and then speeding up again, like the emergency brakes are applies, all the gauges change for e.g the brake pipe/pressure (i think) changes and then slowly creeps back to where it was and once its done that everyhting is back to normal. Does anyone know what the hell is going on and is there any way to stop it/avoid it???