RailMeet 3D

Understand relative speed, meeting time, and complete crossing time.

Ready

Drag to rotate view • Shift-drag to pan • wheel or pinch to zoom • tap a train to track

How to use

1Choose case

2Set trains

3Predict time

4Run & check

Mode

Crossing examples

Train settings

Train A speed72 km/h

Train A length140 m

Train B speed54 km/h

Train B length110 m

Initial clear gap300 m

Trains move in opposite directions

Run & 3D camera

Simulation speed1×

View rotation0°

Camera height50°

Zoom1.00×

Live concept explanation

Approaching

The trains are closing the initial gap. Their relative speed determines how quickly the first engines meet.

Opposite directions: relative speed = speed A + speed B.

Measurements

Elapsed time

0.00 s

Relative speed

35.0 m/s

First contact

8.57 s

Fully crossed

15.71 s

Distance & crossing graph

remaining gapcrossing progress

Object focus

No train selectedFree camera

Position: —Speed: —Length: —Direction: —

Teacher / investigation tools

Train A start delayTrain B start delayPrediction inputSafety margin

Learning objectives

Calculate relative speed for opposite and same-direction motion.
Distinguish first meeting time from complete crossing time.
Explain why both train lengths matter during a full pass.
Convert speeds between kilometers per hour and meters per second.
Use distance-time evidence to verify predictions.

The key idea: relative motion

Relative speed tells how fast one train appears to move from the other train. In opposite directions, speeds add. In the same direction, subtract the slower speed from the faster speed.

Opposite directions: relative speed = speed A + speed B
Same direction: relative speed = faster speed − slower speed
First contact time = initial clear gap divided by relative speed
Complete crossing time after contact = total train length divided by relative speed

Common misconception: “The trains meet” and “the trains completely pass” are different events. The engines meet first; the final carriages clear one another later.

Why lengths are added

After the first engines or nearest ends meet, one train must move past the entire length of both trains relative to the other. Imagine freezing Train B: Train A must cover Train A’s own length plus Train B’s length before the last carriages separate.

The initial gap controls time before first contact.
The sum of lengths controls time spent overlapping.
Speed conversion matters: divide kilometers per hour by 3.6 to obtain meters per second.

Guided investigations

1. First contact versus full crossing

Select Opposite directions.
Run until the engines align, then pause.
Compare elapsed time with First contact.
Continue until the last carriages clear and explain the extra time.

2. Overtaking

Select Express overtakes.
Make both speeds equal.
Explain why complete crossing becomes impossible.
Increase Train A speed and observe how crossing time changes.

3. Long freight train

Keep both speeds fixed.
Double the freight train length.
Compare first-contact and complete-crossing times.
Identify which time changes and why.

Deeper questions

Why does increasing the initial gap not change overlap duration?
Why does doubling both speeds halve all ideal times?
How do start delays change the effective initial separation?
What happens if the rear train is slower in an overtaking problem?
How can a passenger on one train describe the other train’s motion?