Can't help with travel time. It's rush hour.
I'd personally take subway for trek to Penn Station.
For Grand Central, it's only 2 stops from nearest top nearest you hotel using 4,5,6 lines. For me, I like to walk just to get exercise.
The best entrance to get to main Amtrak waiting area is to enter at 33rd Street and 8th Avenue.
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stay in NJ to visit NYC --> [
www.broadwayfocus.com]