The rough distance following a 417 alignment, is only 15 miles. While the tracks are expected to be Class 7 level, with up to 125mph speeds, they aren't likely to use the high-speed trainsets that Brightline would be using (they currently only have a plan for 5 trainsets), but if they were running a "Magical Express" shuttle service they'd probably use commuter-level equipment that's more likely Class 4 and max out at 79mph. But assuming an electric locomotive or EMU setup with good acceleration, I'm going to guess an average of 50mph end to end, we're talking 18 minutes.
Now, obviously a train can handle much more than a bus, but that could mean that dwell times could be longer waiting for a train to arrive and depart after arriving. Plus the trains have to timeshare with the normal Brightline trains, so there will be times when they can't be on the track. AND if the Brightline trains are also stopping...I'm now thinking that it replacing ME is more unlikely as I type...