One thing that people need to understand is that the train is not something new as far a Disney station. The original Florida High Speed Rail system proposed in the late 2000's was to have a 2 level station built to service WDW adjacent to I-4 near World Drive since the train line would be situated within the I-4 Right of Way. The Brightline Rail idea is just an extension of this idea since that project was killed by the Governor in 2014. Disney agreeing to a station is just dusting off the previous plans and saying, sure, we will let you have a stop that services us.
But to think that this line is going to be anything close to a replacement for DME is not even close. These trains are high speed passenger lines, not commuter. There will not be a constant flow of trains moving between the airport and WDW. There may be a few a day going each direction, but the idea is to get people between Miami and Orlando and Orlando and Tampa.
Also - at present, there is not even an approved route to begin a study for environmental approvals (NEPA) let alone a route to begin engineering design and permitting. There are not any agreements yet for use of tracks (they need to have a shared track agreement with CSX to get from the Airport station to the south), or agreements with either the Central Florida Expressway Authority for use of right of way to build tracks along SR 417, or agreements wtih FDOT for co-location of tracks along I-4. THere is a dedicated envelope in the ROW of I-4 for rail, but that was modified for the I-4 expansion project that will soon be moving to construction. I have reviewed the first 2 Brightline proposals for FDOT and there has been nothing new submitted in close to a year, so they are not actively moving on this, other than the agreement to build a station to serve Disney (location unknown).
The station at the airport is being built at Terminal C, which at present is not really in use other than for parking, so you would need to get from terminal A or B with your bags and take the monorail to terminal C to catch a train too. I have been monitoring the construction for the route from the Airport to Cocoa and they are nowhere near ready to lay any tracks. In the past year they have been building several bridges to get the line over major roads, but there are still miles and miles of the route that is just clearing and grubbing and fill dirt with stabiliized support walls. There is no way that the line is running when they have said.