long train tunnels

I can't answer for that line, but you might appreciate this tidbit.

Moo!

About 15 minutes for the Loetschberg Base Tunnel, it is 35 km long.
About 10 minutes for the Simplon Tunnel, it is 20 km long.

What are the chances of a train stopping mid-tunnel? Don't know, but anything can happen.

but rare

I have never known a train to stop in a tunnel (except in an underground métro system in a city). It is as rare as a train stopping on any other section of track between stations. It is never supposed to happen. Only ever happens if there is a technical fault, with your train, the ones in front or the signalling system.
Even if it did, no more issue than stopping anywhere else between stations.

The chance of a mid-tunnel stop southbound seems extremely slim in the Lötschberg, because the northern half is single track and the second half is double track: in other words, trains from the north can enter the tunnel only if the way ahead is clear, because there is only one track. I could imagine a higher risk of stops going the other way, at the point where the double track becomes single track.

Simplon is double track all the way, so very slim chance too.

We took our car through the Lotschberg tunnel on the train and it's a fantastic experience! We went through twice and it didn't stop in either direction and I can't imagine why it would. Very different experience than any other train tunnel I've been through. The trip lasts about 15ish minutes. It's funny to see the people in the cars ahead of you and behind you turning on the overhead lights in their cars to see each other and keep themselves entertained while waiting to come out the other side.

Thank you everyone for your replies. This is very helpful. Sounds like I need to find something else to worry about!
Thanks! James