While there's always a risk for delays (I've been stuck at Heathrow for 3 hours before thinking "If I'd have taken the train I'd be there by now"), you should be aiming for a network at 95%+ of services arriving within a few minutes of the arrival time. I understand that US passenger trains don't have priority, and thus this isn't workable.
On that assumption, given the distance and size of Boston/NY/Philly/Washington, I'd expect at least a 30 minute service, taking well under 4 hours (should really be under 3) from Boston to Washington (with stops in NY, and maybe Phillidelphia). It's only 400 miles, that's like Paris-Marseille, and far shorted than a 4 hour 560 mile Tokyo - Hiroshima trip)