MSL or mean sea level is the average surface level of one or more of Earth’s coastal bodies of water from which heights such as elevation may be measured.
Sometimes referred to as the true altitude, MSL is a fairly constant measurement that pilots (and loco pilots) can read to ensure they are at the correct altitude during navigation.
The precision of altimeters isn’t always perfect. That’s because pilots (and loco pilots) fly over varying landscapes during a flight.
The MSL directly alerts the loco pilots (train drivers) and the guards about the altitude they are travelling at.
With the help of the MSL, a loco-pilot controls the speed of the train. If the train is moving towards a higher altitude the loco-pilots provide power according to the engines for the smooth running of trains.