why are cities across the river from cities on a state line often

[Simfan34]

Yes. Park Avenue's (and by extention, the Upper East Side's) abrupt "end" at 96th Street, where the Grand Central viaducts emerge, makes me chuckle every time I see it. You could be forgiven for thinking the Easy River was there. It's a bit sad though, you wonder what things could have been like if the whole thing had been buried in Manhattan.