Channel tunnel's much harder to pin down, both because it has an economic benefit beyond the revenue from its operation, and because much of the benefit is ~permanent; in a century it will _still_ be producing an economic benefit (really this is a problem with judging the economics of rail infrastructure in general; huge capital outlay, but it lasts a _really_ long time).
The owner company has been close bankruptcy and needed debt restructuring in the past. I don't know what their current figures look like, but there have been special actions to increase utilization in the past, which does not sound like good news. With Brexit things are unlikely to improve, even if we neglect the pandemic as a one time disturbance.
Already in the second year of its operation there was severe fire requiring a closure for 7 months. Remember Twin Towers or Nord Stream. It's a risky project. I don't want to speculate whether it will still be operated in a century, but a lot can go wrong before that.
> I don't know what their current figures look like
2023: Revenue of 1.8bn, EBITDA of 979mn, FCF 638m, 1.5bn in cash. They'll be fine. They're not going to be challenging Nvidia any time soon, but they actually look financially healthy enough for a transport company.
> With Brexit things are unlikely to improve
Interestingly, they may be a rare Brexit beneficiary; their goods traffic seems to be down (as you'd expect) but their tourism traffic is up quite a lot, I suspect because Brexit has made airports more of a pain for British tourists, so taking the train probably looks relatively more attractive. Their revenue is up somewhat now on 2019 (the last non-Brexit-y/pandemic-y year).
> Already in the second year of its operation there was severe fire requiring a closure for 7 months.
Even if it has a major accident, it'll be repaired. For instance see Mount Blanc. Repairing a tunnel, even a badly damaged one, is far, far cheaper than building a new one.
Concorde was probably highly economical, just not directly. It could be seen as the project that started Airbus and saved European aircraft manufacturing. Except for not being able to know how things would have turned out if Concorde didn't exist.
Or, less optimistically, caused Airbus's predecessors' consolidation into Airbus, thus leaving the world with three, and then two, makers of large passenger jets, and leading to the current unfortunate situation where even though the 737's currently kind of a dud, you have to buy it anyway because it's half the market and the A320 side of the market is booked up for years.
Like I think there's a strong argument that the current extreme consolidation of the industry is quite a bad thing.