Can You Measure Train Speed in a Tunnel?
GPS does not work in tunnels. Here is what happens to your speed reading when a train enters a tunnel, and what the alternatives are.
Not reliably. GPS needs line-of-sight to satellites, which concrete and rock block completely. TrainSpeedTest.net freezes or zeroes the reading in a tunnel and resumes automatically once GPS reacquires signal on exit.
- GPS satellites orbit ~20,000 km up — tunnels fully block the signal
- The last known speed is held briefly, then the reading drops to zero
- Browsers cannot access phone accelerometer/gyroscope for dead reckoning
- Long tunnels mean long gaps in the speed chart — this is expected
GPS satellite altitude
Why concrete blocks it completely
Freeze timeout
Before reading drops to zero
Channel Tunnel length
Full GPS blackout for the transit
Gotthard Base Tunnel
World's longest rail tunnel
The Short Answer: Not Reliably With GPS Alone
GPS relies on line-of-sight signals from satellites orbiting around 20,000 km above the Earth. Concrete, rock, and soil block these signals completely. Once a train enters a tunnel, the GPS receiver stops receiving satellite data.
What Happens the Moment You Enter a Tunnel
The transition is not instant — there is a brief grace period before the display changes, followed by a clear signal-lost state.
Phase 1: The Reading Freezes
The last known speed is held for a short period, typically up to the receiver's timeout, often around 15 seconds.
Phase 2: The Reading Drops to Zero
When the GPS timeout expires, the browser's Geolocation API may report a position error or simply stop providing updates. TrainSpeedTest.net will display "Waiting for GPS" and the speed will show as 0.
Phase 3: Erratic Readings on Exit
The first few seconds after emerging from a tunnel can produce inaccurate readings as the GPS re-acquires satellites — this is the same tunnel-exit effect covered in our article on GPS speed jumps.
Could Dead Reckoning Fill the Gap?
Dead reckoning uses an inertial measurement unit (IMU) — accelerometer and gyroscope — to estimate movement when GPS is unavailable. Many smartphones have these sensors, but the browser Geolocation API does not expose IMU data directly. Therefore, browser-based tools like TrainSpeedTest.net cannot currently perform dead reckoning.
Our Approach: Honesty Over Fake Continuity
TrainSpeedTest.net displays "GPS signal lost" or holds the last reading when GPS becomes unavailable, and does not invent fake speed numbers to fill tunnel gaps. Normal measurement resumes automatically once GPS is reacquired after the tunnel.
Long Tunnels Mean Long Gaps
For very long tunnels — the Channel Tunnel at 50 km, the Gotthard Base Tunnel at 57 km — GPS will be unavailable for the entire tunnel transit. There is no reliable browser-based workaround for this limitation.
Reading Your Trip Chart Afterwards
If your train passes through multiple short tunnels, the gaps in GPS data will appear in the speed chart as flat sections or zeroes. This accurately represents the measurement situation rather than indicating a bug in the app.
Frequently Asked Questions
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