Speed Measurement

How Is Train Speed Measured?

Q: How do trains measure their own speed?

Trains typically use wheel-mounted tachometer sensors that count axle revolutions. The rotations per second, multiplied by wheel circumference, gives speed. Modern trains cross-reference this with GPS for accuracy.

Q: Can I use GPS to measure train speed?

Yes. GPS receivers in smartphones can measure train speed accurately. At speeds above 20 km/h on open track (outside tunnels), GPS is typically accurate to ±2–5 km/h. TrainSpeedTest.net uses this technology.

Q: How accurate is GPS speed measurement on a train?

GPS speed measurement via Doppler shift is typically accurate to ±0.1–0.5 km/h on modern devices. Position-based speed calculation (used when Doppler is unavailable) is accurate to ±2–5 km/h under good conditions.

Q: Why does GPS speed reading pause in tunnels?

GPS satellites require line-of-sight to the receiver. Underground tunnels block satellite signals completely. Some railway systems use trackside transponders or inertial sensors to maintain speed measurement in tunnels.

Q: What is the Haversine formula in GPS speed calculation?

The Haversine formula calculates the great-circle distance between two GPS coordinates on Earth's surface, accounting for Earth's curvature. Dividing this distance by the time between measurements gives speed.

From wheel tachometers to GPS Doppler — a complete technical explanation of how railways and mobile devices measure train speed precisely.

7 min read

Try GPS Speed Test GPS Accuracy Guide

Train speed is measured using three primary methods: (1) Wheel tachometers that count axle rotations. (2) GPS receivers using satellite signals and Doppler shift. (3) Trackside transponders and radar systems for signalling.

Onboard tachometers: most common method on modern trains
GPS Doppler: used in smartphones and modern trains for verification
Axle count × circumference = distance per second = speed
Radar transponders: used in high-speed rail signalling systems

±0.2km/h

GPS Doppler accuracy

Modern devices

±2–5km/h

GPS position accuracy

Open sky conditions

24+sats

GPS constellation

GPS + GLONASS + Galileo

<1s

Update frequency

Speed refresh rate

Measurement Methods

4 Ways Train Speed Is Measured

Wheel Tachometer (Most Common)

›A tachometer sensor is mounted near each axle.
›It counts pulses as the axle rotates — typically using a magnetic pickup or optical encoder.
›Speed = (rotations per second) × (wheel circumference).
›For a standard 920mm diameter wheel at 300 km/h: ≈28.6 rotations per second.
›Modern trains use redundant sensors to detect and compensate for wheel slip.

GPS Doppler Speed (Most Accurate)

›GPS receivers measure the Doppler frequency shift in satellite signals.
›As you move towards or away from a satellite, signal frequency changes slightly.
›This shift is proportional to your velocity component towards that satellite.
›Using signals from multiple satellites, a precise 3D velocity vector is computed.
›Accuracy: typically ±0.1–0.5 km/h — more accurate than position-based calculation.

GPS Position-Based Calculation

›Record GPS position at time T1, then again at T2.
›Calculate the great-circle distance between both points using Haversine formula.
›Speed = distance / time_elapsed.
›More susceptible to GPS noise than Doppler — needs filtering.
›TrainSpeedTest.net uses Exponential Moving Average (EMA) to smooth readings.

Trackside Radar & Transponders

›High-speed rail signalling uses Doppler radar to verify train position and speed.
›ETCS Eurobalise transponders embedded in track detect passing trains.
›In Japan, ATC (Automatic Train Control) continuously monitors and enforces speed limits.
›Inertial measurement units (IMUs) supplement GPS inside tunnels.
›These systems achieve safety-critical accuracy of ±1 km/h or better.

How GPS Measures Train Speed (Technical Detail)

Satellite Constellation

Your phone receives signals from 4–12+ satellites simultaneously across GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China) systems. More satellites = better accuracy.

Haversine Formula

For position-based speed: Haversine calculates the shortest distance along Earth's curved surface between two lat/long coordinates. This is then divided by elapsed time to compute speed.

EMA Noise Filtering

Raw GPS data is noisy. TrainSpeedTest.net applies Exponential Moving Average smoothing. Bad GPS samples (accuracy worse than 200m) are discarded automatically.

Privacy & Local Processing

All speed calculations happen inside your browser. No coordinates are transmitted to servers. Your location data never leaves your device.

Measure Your Train Speed Right Now

TrainSpeedTest.net uses the same GPS technology described on this page. Open it on your phone, grant location access, and see your exact speed in km/h, mph, or m/s.

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1. Grant GPS

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2. Board the Train

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3. See Real Speed

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Frequently Asked Questions

How do trains measure their own speed?

Can I use GPS to measure train speed?

How accurate is GPS speed measurement on a train?

Why does GPS speed reading pause in tunnels?

What is the Haversine formula in GPS speed calculation?

Related Train Speed Resources

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Technical

GPS Accuracy Explained

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