FEC does provide access to two Class 1's for the South Florida intermodal business, including the international container trade. Their financial relationship with both NS and CSX and across the various "bill-to" parties likely has some variety across the range.
Port of Miami container volume has been up at least into FY21, half of it S. American trade venturing that, in general terms, their market beyond FL is the Southeastern US with Atlanta a good part of that. To the extent of the port's participation in the South Florida rail intermodal market as 1 of 2 container ports, besides likely generating some movement of containers they are also likely generating backhaul truckload and less-than-truckload volumes transloaded into trailers and domestic containers that would otherwise move empty.
Consequently, given the port's probable role in the rail market and mix of equipment serving the market, I suspect it makes more sense to better utilize capacity within blocks and trains to serve the port via the FEC ramp, at least to a greater extent.
As an aside to the role of ports, the transportation demand they generate, how that transportation demand is met, and all within supply chains, as imports via rail have evolved from exclusively moving in international containers to then transloading into domestic containers to fill empty backhaul capacity as well, with the rise of e-commerce we now also have direct-to-consumer retailers, often located near ports, using small-shipment providers to reach their markets. In South Florida, that port/e-commerce dynamic goes to furniture (flat-pack) and tile.