Open a GPS speedometer on a train and it usually works just as well as it does on the ground — right up until the train enters a tunnel, at which point the reading freezes or disappears entirely. Both behaviours come down to the same simple fact about how GPS signals travel.
Why GPS works through glass but not through tunnels
GPS satellites broadcast on a frequency around 1575 MHz, in the microwave band. Ordinary window glass is largely transparent at that frequency, which is why a phone sitting on a table next to a train window can usually still get a solid satellite lock — the signal passes through the glass with only minor attenuation, much like visible light does. A metal train roof, a concrete tunnel, or a mountain of solid rock is a completely different story: none of those are transparent to microwave signals at any useful level, so the satellite signal simply can't reach the receiver at all.
This is also why a window seat matters more on a train than almost any other vehicle. Carriage roofs are typically metal, which can shield a receiver from satellites directly overhead even when the train is out in the open, making a clear side view through the glass the most reliable path to a satellite lock.
What your speedometer does when signal drops
When a GPS receiver loses its satellite lock — entering a tunnel being the most common cause on a train — there's no new position data coming in, so there is nothing to calculate a fresh speed from. Most GPS speedometers, including this site's train speed checker, handle this by holding the last known reading briefly and then indicating that signal has been lost, rather than inventing a plausible-looking number. As soon as the train re-emerges into open sky, a fresh satellite lock typically takes just a few seconds to re-establish, and live readings resume.
How fast do trains actually go
Speeds vary enormously by service type. A typical urban commuter train cruises around 60 to 100 km/h between stations. Regional and intercity services on conventional track often run at 120 to 160 km/h. Dedicated high-speed rail — the TGV in France, the Shinkansen in Japan, ICE in Germany — regularly operates at 250 to 320 km/h in commercial service, and test runs on some of these systems have exceeded 570 km/h. Watching a GPS speed checker climb past 250 km/h on a high-speed line is a genuinely striking way to see those numbers made real, rather than just reading them on a spec sheet.
Getting the best accuracy on board
For the most reliable reading: take a window seat rather than an aisle seat, keep the device reasonably close to the glass rather than buried in a bag, and don't expect instant signal the moment you sit down — allowing 5 to 10 seconds for an initial lock applies on trains just as it does anywhere else. Expect brief, normal interruptions through tunnels and deep cuttings, and don't be surprised if the reading is rock steady the rest of the journey.
The takeaway
GPS on trains works reliably whenever there's a clear path to the sky through glass, and fails predictably and briefly whenever that path is blocked by metal, concrete, or rock. Neither behaviour is a fault in your phone — it's simply physics. Try the train speed checker on your next journey and you'll see both behaviours play out in real time.