I don’t think there is anything wrong with changing the schedule. Anything else would be quite dumb and probably just delusional. Perfectly predicting the future is not possible and if honest mistakes or simply unknowns lead to this then that’s just normal.

However, here “ahead of schedule” is used as a, well, I guess compliment and for that it’s important to know the context. That they adjusted the schedule is obviously not at all a problem and, as I said, for a project this big the couple years it took longer aren’t really that relevant even.

And, more often than not, these requirement changes are used as excuses for extending the deadline so that project ends up being done "on time". I've worked in management on multi-year waterfall projects and this is how this game is played.

It would be fun to see an "agile" tunnel project.

"This rock is too solid to drill through with our machines!" – "Ok, for now let's just drill around it and make it an underground rollercoaster instead of a railway, we can get back to the original design later!"

This actually happened in St. Petersburg, when they ran into difficulties tunneling under the river on Line 2. They ended up going under the difficult section, and now there's a 6% downgrade on that section of the line, which is pretty much a rollercoaster by subway train standards.

That's not agile, that's a pivot :)

Not in this case, as the train is specified to go at 250 km/h, which means that the tunnel had to be pretty straight, or else the train might derail at that speed.

Which would make it an even more fun roller coaster!

Just like the UK 2012 Olympics. It came in under budget. Despite the budget quadrupling since the project was started.