Technique
What a train covers when it crosses something
Arithmetic • Time, Speed and Distance • Trains
Statement
A train crossing an object covers its own length plus the length of that object: a pole or a standing man adds nothing, a platform or a tunnel adds its own length, and a second train adds that train's length, with the whole distance covered at the relative speed.
How to use it
- 1Write the distance as the train's length plus the length of what is crossed, then divide by the relative speed.
- 2Crossing a pole gives the train's own length directly, so that event is the cheapest one to use first.
- 3For two trains the distance is the sum of both lengths whichever way they move; only the relative speed changes.
Worked example
A m train passes a pole in s. How long does it take to cross a m platform?
Step-by-step
- 1Speed m/s
Why: Crossing a pole covers the train's own length only.
- 2Crossing the platform covers m
Why: Its own length plus the platform's.
- 3s
Answer
seconds
Watch out
Quoting the train's length when the platform's was asked, or the whole covered distance as the platform length, are the two standard slips. 'Completely crosses' another train means both lengths have gone past, not one.