Trains Problems

Relative speed is the speed of one object as seen from another moving object. If two trains move towards each other, they are moving in opposite directions. You must add their speeds to find how fast the gap closes. If they move in the same direction, the faster train must 'catch up.' In this case, subtract the smaller speed from the larger speed. This concept is essential when two trains cross or overtake each other.

The 'Distance' in the formula is the total path the train covers to fully pass an object. If a train passes a stationary man, the distance is just the train's length. However, if it passes a tunnel or platform, the train's front enters and the back must exit. Therefore, the total distance is the sum of the train's length and the stationary object's length. If two trains cross each other, the distance is the sum of both train lengths.

In train problems, speed is usually given in km/h while length is in meters. You must make them consistent. To convert km/h to m/s, use the fraction 5/18. For example, if a train moves at 72 km/h, its speed in m/s is 72 multiplied by 5/18. This equals 20 m/s. Converting first prevents huge calculation errors later in the problem. Always look at the units of the final answer options before starting your work.