Oh man, that's an easy one!
It'd have to be my home area, Northern New Jersey, say from 1945 to 1955. All those 'roads, the New York Central, the Pennsylvania, the Erie, the Lackawanna, the New York Ontario & Western, the Reading, the Jersey Central, the Lehigh Valley, the Susquehanna, plus lesser 'roads like the Raritan River and the Middletown and New Jersey, most converging on the New York area, plus the added attraction of all those railroad marine fleets of ferrys and tugboats. Steam to diesel transition era, but still quite a bit of steam left by 1955. Non-stop action and a railfan's heaven, it had to be, considering all the photographs and amateur movies shot by so many at the time. And you just might have run into legendary master railfan photographers like Bob Collins and the great Bob Malinoski at trackside!
Mind you, it would have to be the immediate post-war era. During the war you didn't want to be caught dead by a busy rail line with a camera unless you had some friends on the railroad.
I mean, you sure didn't want anyone to think you fit into this category!
https://www.youtube.com/watch?v=Y4M0BW1GLnw