Before certification there were plenty of stretches on the New Haven Line where experienced engineers drove their trains in excess of the speed limits. And there are plenty of areas where the speed could be increased if the tracks were maintained to a higher standard. But there are so many improvements to bridges ( movable and ballast deck) and I believe the constant tension catenary project is still yet to be completed and, if PTC is not yet operational, the use of the cab signal system to enforce civil speed restrictions just makes it impossible to take any time out of schedules. As Ducth alluded, all it would take is money and the will to do it. But never say never, Acelas would never use their tilt on MN,until they did, and the possibility of MN trains running to Penn Station would surely encourage more co operation between AMT and MN.