I love the double letter trains. And curious to know how/why certain trains seemed to switch routes. The 1972 map shows the 3/4 stopping at Brooklyn College; now it’s the 2/5. But in ‘72 the 2 went to New Lots Ave., which is now where the 3 goes. Confusion!
The #1 train is pretty much why and when the 2 and 3 changed routes. You’ll notice that the 2 and 5 go one way and the 3 and 4 go another after the trains come above ground.
I used to live where there was a GG train, which became the G. No change in route. I also seem to remember an AA train, which - I think - became the C line.