Train and road trips: gaps are normal, so plan around them

Speed forces constant handovers between masts and the carriage shell attenuates signal; together they produce frequent gaps.

Yes, provided you download the maps first, since offline navigation does not need constant signal.

No. Price has nothing to do with how many masts stand along your route. What differs between plans is the allowance, the validity and the list of countries covered. If your line runs through thinly covered country, no plan changes that fact; offline maps and material already downloaded to the device are what actually help.

It is worth it when you know the stretch is long and genuinely dead, because it stops the battery being spent searching for a network that is not there. For short or uncertain gaps, leave the phone as it is: you might otherwise miss messages that arrive during a brief return of signal, and it is easy to forget to switch airplane mode off afterwards.

It depends on how that rail operator issues tickets. Some produce a file or code you can save and present offline; others require an app that validates against a server. The safest habit is to open the ticket and save it as a file or screenshot while still on Wi-Fi, and to check that operator's own terms.

If that country is on your plan's list, everything continues as normal. If it is not, data stops until the phone moves back to a covered network. Toggling airplane mode once when you are clearly back inside the right country usually speeds up the re-registration.

When the route crosses several countries, even ones you only drive through, a regional plan usually fits better: no swapping profiles mid-journey and no stretches where data quietly stops. Compare each plan's country list against your actual route before deciding.