In both NYC and Boston, the track for boarding isn’t shown to the waiting area until arriving passengers had a chance to leave the train. Once your track number is shown you have only a couple minutes to board, and depart.
This means you can arrive at NYC’s Penn Station or Boston’s South Station 5 minutes before departure and walk right onto your train.
When I factor the terminal location and the boarding convenience at both ends of my trip, I can get door to door from midtown NYC to downtown Boston faster by Acela than by air.
Before TSA, the flights won. I could hit LGA curbside 15 minutes before gate close and be in my plane seat. Now, the variability is much too high, and TSA is too likely to be completely mismanaging surge situations, so typically 2 hours buffer is needed as a guarantee instead of 15 mins.