by The EGE
Intercity equipment in the US isn't as standardized on bilevels as is the commuter market, Brightline is an intercity service that doesn't necessarily need the capacity of bilevels. Additionally, single-level equipment puts 100% of seats at platform level with no stairs, which might be handy given Florida's aged and aging population. Additionally, the current issues with manufacturing bilevel intercity equipment for the US market means bilevels might not have suited their timeline.
More speculatively, I think that the endgame for FEC might be to sell the service to Amtrak after five or ten years. It's worth it to FEC's holding company to construct as a loss leader for their real estate holdings, but once the service is established and the real estate sold or leased, it doesn't particularly matter to them who runs the service. After five or ten years, with the legal issues long put away and the demand steady, Amtrak might well be persuaded to add it to their national system, especially with the connection at Jacksonville. And that transfer gets a lot easier if Brightline has the same locomotives and very similar passenger cars to Amtrak.
More speculatively, I think that the endgame for FEC might be to sell the service to Amtrak after five or ten years. It's worth it to FEC's holding company to construct as a loss leader for their real estate holdings, but once the service is established and the real estate sold or leased, it doesn't particularly matter to them who runs the service. After five or ten years, with the legal issues long put away and the demand steady, Amtrak might well be persuaded to add it to their national system, especially with the connection at Jacksonville. And that transfer gets a lot easier if Brightline has the same locomotives and very similar passenger cars to Amtrak.