Answer:
the total distance covered by the train
Explanation:
-the direction doesn't matter, a train can go 40 km/h in any direction
-the mass doesn't matter, a heavy or a light train can go 40 km/h
-the time doesn't matter, it can be travelling at 2 am or 9 pm, that doesn't influence the speed.
The formula for velocity in physics is V = S/T (velocity = space divided by time). So space here would be the distance covered, which is the missing component. V = 40km/h, the time will be determined by the distance covered. Ex: if the distance covered is 80 km, then the time will be 2 hours.