坐火车穿越欧洲的 eSIM

Generally not, provided both countries sit within the same plan's coverage — the phone moves onto a network in the new country by itself. What you should do is check before boarding that every country on the route is genuinely on the coverage list.

No. In a tunnel or anywhere with no network to attach to, every kind of SIM is equally unusable. Once you are out, the phone reconnects on its own. If it does not within a reasonable time, cycle airplane mode once.

For a rail trip crossing several countries a regional plan is usually easier to manage, because you are not changing profiles mid-journey on a train with an unreliable signal. Just confirm first that all four countries really appear on that plan's coverage list.

Prepare for that stretch as you would for no signal: save tickets, maps and anything else you need offline before entering. Or consider a different plan that covers the whole route, if you will be spending a meaningful amount of time there.

Installing requires an internet connection, and on-board signal comes and goes, which makes a train a poor place to attempt it. Install on reliable wi-fi before you travel. If you genuinely forgot, wait for a station or accommodation with a steadier connection.