Atrain is accelerating at a rate of 2 m/s. if it's initial velocity is 20 m/s, what is its velocity after 30 seconds?

We don't know anything about the direction of the track, so we can't say anything about the train's 'velocity'.  The best we can do is talk about its speed.

Speed = (initial speed) + (acceleration) x (time spent accelerating)

Speed = (20 m/s) + (2 m/s²) · (30 sec)

Speed = (20 m/s) + (60 m/s)

Speed = 80 m/s

(This is about 179 miles per hour, so if this is not a bullet train of some kind, then get ready for an ugly crash.)

explanation:

newton's third law, for every action there is an equal and opposite reaction.

vapour or gas assuming atmospheriv pressure