The Acelas and Northeast Regionals now handle 11 million passengers per year. In 10 years they could be handling 16 million passengers per year. The majority of those additional 5 million passengers won't be coming out of aircraft, but out of cars and buses.
So how do you build a system to handle another 5 million passengers per year? First of all, you need the tracks and signals to carry them. That means replacing movable bridges with high level bridges where possible, adding track at bottlenecks, upgrading signals, replacing old catenary poles, wires and substations.
Secondarily, you need equipment. New engines (on order) and new passenger cars. Passenger comfort while riding should exceed anything offered by air, bus or driving.
Thirdly, you need to get them on and off the train. Station dwell time can crush a schedule. So find ways to get 40% more people on and off the train in less time. It may require wider doors or wider platforms, more parking or a change in operations. How do we get passengers through New York, through Philadelphia and through Washington with less delay?
And when it comes to speed, since most of those new passengers are coming from cars and buses, focus on those areas where current operating speeds are less than 70 mph. Get those areas up over 70 and people will be flocking through the doors.