A train travels at 40 mph. If it takes 6 minutes to pass you, a stationary object, how long is the train?

Answer:
Train takes 6 min to pass you. That means that if the 
head (engine) of the train crossed you at some time, the end of
the train crossed you 6 min later. 

In the same 6 min, the how far would the engine of 
train move? Obviously the length of the train.

So, in the 6 min that it takes to cross you, the distance
travelled by the train is nothing but the length of the
train.

Distance travelled in 60 min = 40 miles.
Therefore distance travelled in 6 min = 40*6/60 = 4 miles

So the length of the train is 4 miles.

That's a pretty long train :)