1. You can't have the airport in the middle of the city. Add 20-45 minutes to arrive / leave at each one. 2. Airport security. Add 15-45 minutes depending on d

1. You can't have the airport in the middle of the city. Add 20-45 minutes to arrive / leave at each one. 2. Airport security. Add 15-45 minutes depending on density. 3. Delays. Frequent & much more common with airlines than trains. (add 1-several hours every hundred flights or so). 4. Capacity. Need 100 more passengers capacity? Add another carriage to the train or schedule an entirely different flight. 4. Also add in that you're wanting to cram in a bunch of airports into a few hundred miles of each other. You have to take into account that planes are moving at 550+ mph and need lots of clearance around each one. It's a lot easier to even schedule another train 10 minutes behind an existing one than it is to schedule another flight (considering the variables of destinations, companies and other logistics involved). There's a reason why ATC jobs are some of the most stressful in the world. Flying seems easy, but the logistics behind it are intense.