Thanks to 7 Train for finding on nyscsubway.org the 1907 date. Here is another inconclusive comment: Some engineer designed a few of the stations on the #1 line with grades entering and leaving the station. I believe a train entering 157th, 168th, and 181st goes up a hill when entering (designed to assist braking) and goes down a slope leaving (designed to assist acceleration). Of course, after WWII, the stations were lengthened, but the ramps left in place. The next time I ride past 191 Street, I will have to see whether it is similarly atop a small incline. If not, it might suggest (as my uncertain memory says) that 191 St Station was an afterthought. The only other system where I notice grades entering stations is Montreal (again line 1), but that was for a different reason: to have the stations less deep than the bored tunnels between stations.