Well,this question has tricky part that may confused the person who answered that it is part of the equation but it is not obviously.The trains length may some sort kind of a trivia thing....
Well here's the equation for you to know the answer.
[tex] \frac{20m.}{1s.}\times60s.\to20m\ \cdot\ 60\to\boxed{1,200\ meters} [/tex]
Conclusion;
Thus,1 200 is the length of the tunnel.
Hope it Helps:)
-----Domini-----