I don't have a clear memory how I first-first heard about it. I had a friend to worked for Skanska in legal, and they mentioned it, but I might have also heard about it through the 2nd Avenue Sagas blog. The criteria was that you had to live within 1 avenue block of 2nd Avenue to qualify. I did, but much further south of the construction zone. They didn't care.

Either way, I emailed somebody, they added me to a list, and they were really good about updating people about when tours were available (all options were early morning Saturdays, if I recall), and what would be expected of them (wear clothes that can get dirty, shoes you really don't care about, you'll end up walking a couple miles before the tour is over, but not a lot of stairs because it's a mine-shaft-ish elevator sto get in and out. You'll meet at this location near 71st st for the introduction, safety lesson, and to issue gear, the tour will walk you over from this station to the next one, expect to get "rained" on, and we'll come back up there instead of the way we went in...

The second tour I think was available only to people who'd been on the earlier tours. By this point it was almost a complete station. They were still putting up tiles and doing wiring, but things were dry, painted, and clean. The Bronx Borough president was on our tour, so there was a gaggle of photographers along with, but they basically ignored us normies.

Both tours were awesome in their own way, but I liked the first one better, and I had a lot of fun remembering all this stuff for this comment. If you ever are near an MTA contstruction zone, ask around to see if they're doing a tour.