How the heck to you add 3 to 4 hours above the flight time between arrival and departure on a domestic flight? Especially recently. Even parking off site, I arrive 1 1/2 hours before departure if I'm checking bags, less if I'm only doing carry on. Even getting a rental car upon arrival takes maybe 15 minutes and my wife can wait for the bags while I'm at the counter. Less than 2 hours is added to the flight time.
Heck, last week I flew for business and just beat the 45 minute deadline for checking bags and walked to the gate just as boarding began. The airports were pretty empty (not quite as empty as May 2020) so that can only be done under specific circumstances.
With a train you can't just show up right at the scheduled time and leave the station instantaneously. Sure it will take less time than with flying but probably more like an hour less. The actual trip time will be shorter by air than train. The longer the distance the more time is saved en route.
For my house in south Florida to WDW, door to door the fastest way (with or without this eventual train route) is driving. It takes me a little over 3 hours including a restroom stop at one of the turnpike plazas. Even though the flight is only around 45 minutes, it takes me 20 minutes to get to the airport, another 15 to shuttle from parking to the terminal, I have to be there an hour early, 15 minutes to get a rental car and close to a half hour to get from MCO to WDW. Not much longer than driving but a few minutes longer.
Brightline even with a station in WDW will take longer door to door than either current option. Plus, I want my vehicle when I'm there so if I drive that's taken care of. I tend to stay off site and eat mostly off site so that's a big consideration for me.