As far as I remember, the whole 'turn off your phone on a plane' was just a precautionary measure and is not a real technical problem nowadays.
The risk was that mobile networks could not handle moving many devices from one cell to another at high speeds (during takeoff and landing).
I think part of the issue is that cell tower antennas are designed for talking to devices on the ground or at very low altitudes (like those you'd experience in a tall building). So a cell tower's capacity for talking to lots of somethings directly above it, thousands of feet up, is much lower than talking to lots of somethings below it or adjacent to it.
As far as I remember, the whole 'turn off your phone on a plane' was just a precautionary measure and is not a real technical problem nowadays.
My memory is that it was necessary at the time when lots of people started taking phones on airplanes because the wiring/navigation wasn't shielded against a transmitter that might be actually inside the aircraft.
Since then, plane electronics are better insulated making it less of a problem.
There are another two issues that aren’t technical, which are starting to come up again now that Internet access is rapidly becoming available and good onboard aircraft:
- People not paying attention to/ignoring the instructions of the FAs during safety briefings and emergencies due to being engaged in a phone call.
- People being assholes and talking on the phone, bothering the person stuck in the seat next to them.
On all of the flights I’ve been on recently the preflight brief has been crystal clear that you can do whatever you want on the internet connection except have voice calls.
How would that be different for trains? Trains would have similar numbers or more devices, moving at a similar speed (for high speed trains compared to planes at take-off/landing).