The topic "problems on trains" is an application extension of time ,speed and distance. hence uses the same formulae.
but there are certain things which should be kept in mind before solving these kind of questions.
1.when there is an asking of time required to cross the pole it means that time required to pass the distance = length of train.
2. when there is an asking of time required to cross the another train (or a platform) then total distance is taken as length of train + length of another train (or the length of platform).
3. when trains are moving in the same direction on parallel tracks then relative speed =
4. when trains are moving in opposite direction the relative speed = speed of train 1 + speed of train 2
5.Assume that two trains (objects) start from two points P and Q towards each other at the same time and after crossing they take p and q seconds to reach Q and P respectively. Then,
A's speed: B's speed =
6.Assume two trains of length x meters and y meters are moving in opposite directions at v1 m/s and v2 m/s, Then the time taken by the trains to cross each other = (x+y) / (v1+v2) second.