How Do Trains Measure Speed?
Inside the cab, trains use tachometers, radar, GPS, and balise transponders to know exactly how fast they're going — here's how each system works.
Trains measure speed primarily through axle-mounted tachometers that count wheel revolutions. Speed equals rotations per second multiplied by wheel circumference. Modern trains cross-reference this with GPS and, on ETCS-equipped lines, trackside balise transponders for accuracy. Doppler radar provides an additional independent measurement.
- Axle tachometer: counts wheel revolutions per second
- GPS verification: cross-checks tachometer data
- ETCS balises: fixed reference points calibrate odometry
- Doppler radar: independent ground-speed measurement
Standard wheel diameter
European passenger trains
Tachometer accuracy
When properly calibrated
Measurement latency
Safety-critical response
Redundant sensors
Multiple axles monitored
Axle Tachometers: The Primary Method
The backbone of train speed measurement is the axle tachometer — a sensor mounted on one or more axles that counts wheel rotations.
How it works
A toothed ring or magnetic encoder disc is fixed to the axle. As the wheel turns, teeth pass a fixed pickup sensor, generating electrical pulses. The pulse frequency is directly proportional to rotational speed.
Converting to linear speed
Linear speed = rotational speed x wheel circumference. For a standard 920 mm diameter wheel (circumference 2.89 m), 28.8 revolutions per second equals 300 km/h.
Wheel wear problem
As wheels wear, diameter decreases by up to 80 mm over their service life. A worn wheel (840 mm) rotates 9.5% faster than a new one (920 mm) at the same ground speed, causing the tachometer to over-read by 9.5% if uncorrected.
Calibration
Regular calibration corrects for wheel wear. Modern systems auto-calibrate using GPS data or ETCS balise positions. Manual calibration involves measuring wheel diameter and updating the onboard computer.
GPS Cross-Referencing
GPS provides an independent speed reference that helps validate and calibrate tachometer readings.
Doppler-based GPS speed
Railway GPS receivers use Doppler shift measurements from satellite signals to calculate velocity. This gives speed accuracy of ±0.5–2 km/h independent of wheel condition.
Limitations
GPS is not safety-certified on most railways because signal loss in tunnels and urban canyons makes it unreliable as a primary speed source. It serves as a calibration and verification reference instead.
Integration
The onboard computer continuously compares GPS speed with tachometer speed. Persistent discrepancy triggers a wheel-wear alert and automatic recalibration.
ETCS and Balise-Based Calibration
On European Train Control System (ETCS) equipped lines, trackside transponders provide precise location references that improve speed accuracy.
Eurobalises
Passive transponders embedded between the rails transmit a fixed position telegram when energized by the passing train's antenna. The onboard computer knows the exact distance between consecutive balises.
Odometry correction
By comparing the tachometer's measured distance between two balises with the known actual distance, the system calculates and corrects any wheel-diameter error in real time.
Speed supervision
ETCS uses the corrected speed to enforce speed limits. If the train exceeds the permitted speed, the system applies brakes automatically — first a warning, then service brake, then emergency brake.
Doppler Radar
Some trains carry Doppler radar units that measure ground speed independently of wheel rotation.
How it works
A radar beam directed at the ground reflects back. The frequency shift between transmitted and reflected signal is proportional to the train's ground speed — unaffected by wheel slip, spin, or wear.
Where it's used
Doppler radar is common on high-speed trains and in safety-critical applications where wheel-slip could compromise tachometer readings, such as during heavy braking or acceleration on wet rails.
Frequently Asked Questions
Related Train Speed Resources
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