You could easily build in emergency-use-only, slow-speed-required exit valves..

I kinda can't imagine them not having something comparable to the safety mechanisms in the Chunnel...

Basic repairs, emergencies, etc can't require a full service stop across the line. There are solutions used in similar situations that are baseline operational requirements.

Passengers still get trapped for hours on airplanes sitting on tarmacs. I feel like there's a couple of stories about that per year, and that's just what gets reported in the media.

No, they don't, not any more.

https://www.nolo.com/legal-encyclopedia/tarmac-delays-airlin...

Even back in the day when it did happen, it was because of policy, not physical constraints.

Three, or two, hours still counts as 'trapped for hours'.

Airplanes are not comfortable at all, and no hyperloop pods actually exist - that statement makes no sense.

Except that airplanes are designed to be occupied for hours. In particular, airplanes have bathrooms.

AIUI airplanes just sitting on the tarmac don't have AC (because they're not running their engines).

AIUI?

(But this is another good point: a failed hyperloop also won't have A/C. That will be a potentially life-threatening problem in Dubai, where the first hyperloop is likely to be built. Summer temperatures there regularly reach 45C.)

AIUI means As I Understand It.

Why can't a failed hyperloop have A/C?

Ah.

So first, commercial aircraft typically start their engines immediately upon leaving the gate. An extended period of time without A/C is extremely rare, and nowadays regulations require A/C for any tarmac delay over an hour.

Second, the reason you can't have A/C in a failed hyperloop is that there's nowhere to discharge the heat. As long as the pods are moving you can discharge heat into the tube (because it's not a hard vacuum) but as soon as they stop you can't do that any more.

Discharging heat is a good point. I imagine you could probably design the pods such that if they break down, they could extend radiators to contact the hyperloop wall in order to discharge heat that way.

Though actually this raises another question in my mind. How do spacecraft handle heat? Or is the problem with spacecraft keeping them warm enough, rather than dealing with excess heat?

Spacecraft use radiators to radiate excess heat into space. Depending on how close they are to the sun they can have both heating and cooling problems simultaneously. The side facing the sun can get too hot and the side away from the sun can get too cold. Thermal management of spacecraft is an engineering discipline unto itself. (I used to work for NASA.)

Radiating heat to the tube won't work because the tube is either above ground, in which case it's likely to be much hotter than ambient if the sun is up, or underground, in which case you've effectively wrapped the tube in a nice insulating blanket of dirt, and again there is no place for the heat to go.

I mean, you could build an active cooling system for the tube, and start it up whenever you have a broken-down pod radiating heat into the tube, though that seems like a lot of effort / complexity.

Maybe the focus should be on how to get people out of a broken-down pod within a reasonable period of time rather than cooling said broken-down pod. Unfortunately I don't have any ideas on this front.