How to Solve Trains Crossing a Platform Problems
Solve trains-crossing-a-platform aptitude problems using the train-plus-platform-length rule, unit conversion, and a worked example with practice questions.
Expected Interview Answer
When a train crosses a platform, the distance covered equals the train length plus the platform length, so time = (train length + platform length) / speed, with speed converted to consistent units first.
The key insight is that “crossing” is measured from the moment the engine (front) reaches the platform start to the moment the last coach (rear) clears the platform end, so the total distance traveled is the sum of both lengths, not just one. Contrast this with crossing a pole or a stationary point, where the distance is only the train length. Always convert km/h to m/s by multiplying by 5/18 when lengths are in meters and time is in seconds, since mixing units is the most common source of error. Once distance and speed are in matching units, time = distance / speed gives the answer directly.
- One formula covers platforms, bridges, and tunnels alike
- The train-length-plus-object-length rule prevents the most common setup mistake
- Unit conversion (km/h to m/s) becomes a reliable habit
AI Mentor Explanation
Think of a fielder sprinting from the boundary rope to field a ball and then running back to the rope with the ball in hand — the total ground covered is the distance to the ball plus the distance back, not just one leg of the run. A train crossing a platform similarly covers its own length plus the platform length before it is fully clear, because “crossing” ends only when the last coach exits, not when the engine enters. Timing the fielder from the start of the sprint to the return at the rope mirrors timing the train from when the engine reaches the platform to when the rear leaves it.
Worked example
Convert speed
- 54 km/h × 5/18
- = 15 m/s
Total distance
- 150m + 250m
- = 400m
Time
- 400 / 15
- ≈ 26.67 s
Step-by-Step Explanation
Step 1
Identify both lengths
Total distance = train length + platform (or bridge/tunnel) length.
Step 2
Convert units
Multiply km/h by 5/18 to get m/s when lengths are in meters.
Step 3
Apply time = distance/speed
Divide the combined length by the converted speed.
Step 4
Sanity-check
Crossing a platform always takes longer than crossing a pole at the same speed.
What Interviewer Expects
- Correct identification that distance = train length + platform length
- Accurate km/h to m/s conversion using 5/18
- Distinguishing platform-crossing from pole-crossing setups
- Clean final division to get time in seconds
Common Mistakes
- Using only the train length and ignoring the platform length
- Forgetting to convert km/h to m/s before dividing
- Mixing up multiplying vs dividing by 5/18 (should multiply km/h by 5/18 for m/s)
- Confusing platform-crossing time with pole-crossing time
Best Answer (HR Friendly)
“Whenever a train crosses a platform, I add the train's length to the platform's length to get the total distance traveled, because the crossing only finishes when the last coach clears the platform. Then I convert the speed to meters per second using the 5/18 factor if it is given in km/h, and divide the combined distance by that speed to get the time. It is the same logic as crossing a bridge or tunnel — always train length plus obstacle length.”
Follow-up Questions
- How does crossing a stationary pole differ from crossing a platform?
- How would you find the platform length if train length, speed and time are given?
- What changes if the train is crossing another moving object instead of a platform?
- How do you convert m/s back to km/h for a final answer?
MCQ Practice
1. A 200m train running at 72 km/h crosses a 300m platform. Time taken is?
Speed = 72×5/18 = 20 m/s. Distance = 200+300 = 500m. Time = 500/20 = 25s.
2. A train crosses a 100m platform in 20 seconds and a pole in 10 seconds, both at the same speed. The train length is?
Speed = train length/10. Platform: (train+100)/speed = 20 → train+100 = 2×train → train = 100m.
3. For a train crossing a platform, the total distance traveled equals?
Crossing ends only when the rear of the train clears the platform, so distance = train length + platform length.
Flash Cards
Distance formula for crossing a platform? — Distance = train length + platform length.
How to convert km/h to m/s? — Multiply by 5/18.
When does “crossing” end? — When the last coach (rear) fully clears the platform.
Formula for time? — Time = (train length + platform length) / speed.