NYC to Pittsburgh is actually a subpar trip in terms of Northeast train travel. It's smooth-sailing from NYC to Philly and Philly to Harrisburg, but once you get west of Harrisburg, speeds drop drastically and the resulting trip is much slower than driving. This is because the tracks are shared with other trains and run through mountainous terrain. Because the trip isn't great, it's not that heavily used and Amtrak only runs one train a day each way between Pittsburgh and Harrisburg.

It's a shame really! A high speed track between Pittsburgh and Harrisburg would really improve accessibility for the city and be well used I think. I know there's been some talk of it, but no real progress made.