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.
"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.
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.
To all you western europeans complaining: come down to Brazil, and I'll take you on a tour of our "Top Ten Most Abandoned-Halfway Billion Dollar Government Works" ;-)
Oh, and just to give an example, the construction of one of our nuclear power plants began in 1984 but was halted in 1986, only to begin where it left off in 2010, to be finished in 2015. Wait, now it is in 2016. Nope, just checked, it should be in 2018. All the while, the cost to maintain the stuff that was already bought or built is around 20 million a year, since 1986. So yeah, this one is on our TOP TEN list. This little swiss tunnel is not.
Well construction of the Cologne Cathedral started in 1248 and was halted in 1473, only to begin where it left off in 1842 and was eventually completed in 1880 - so don't give up hope ;)
It's even better. They are about one year ahead of schedule.