Brightline does not even connect with either airport as the stations are in Downtown FLL and Downtown Miami. As stated in AStarr's post, tri-rail has a shuttle that runs from FLL to the station at Griffin Road which you can board and take it to MIA where the station is at the Intermodal Center. You then will have to go upstairs and take the MIA Mover to the center of the Concourses near E/F on the third level, so you will have some walking to your check-in area once you arrive at MIA.

You can always grab the GO-shuttle curbside or make a reservation with SuperShuttle to take you to MIA.

MIA to FLL, you can grab Supershuttle curbside at MIA, or do the reverse by taking the MIA Mover to the Intermodal Center and then grab the Tri-Rail to the Griffin Road/FLL Airport station take the shuttle to the arrival level at FLL.

I wonder if the poster ever made the MIA/FLL connection. I can't see it even under the best conditions. Possibly if the OP had GE, no checked bags, got an uber right away and the Express Lanes were fully open and not clogged. Or his BA flight was extremely early and he had GE.

Have things changed and now that Brightline apparently runs from downtown Miami, is there a convenient transit link between the Brightline station in downtown Miami and the Miami airport?

I've searched online and it looks as though Tri-Rail serves both the airport and downtown Miami, and Brightline runs from downtown Miami, but I'm not clear on how easy this is in real life, particularly on a weekend. I have a connection between MIA and Brightline tomorrow and would appreciate any insights as to how to make the trip between MIA and Brightline, and how easy it is in real life to use transit.

Thanks.

Simply take the Miami Metro Rail from MIA airport. Follow the signs to Rental Cars and transportation hub (Tri-Rail, Miami Metro Rail and Metro Bus). Take the Metro Rail to the Overton/Lyric Theatre stop and walk downstairs to the Brightline. Fare is $2.25 payable by Metro Card you can get at the kiosk. Word of warning the machines aren't very good at taking credit and debit cards so have a little cash on you.

The purchase of a "smart" card is now mandatory -- there's no other legal way to ride the train. The fare is also higher on weekends -- $7 total for new customers.