So, if these were tests with 2 locos and 10 cars....
It's pretty clear they could back the train in, park the locomotives across the crossing, and add 2 more cars. 3 more if the people in the front car had to walk back to the second car to exit, which has been required at Syracuse NY (among other places).
It's unlikely that the Star or Meteor will exceed 13 cars any time soon. So that gets you to 13 cars.
Backing in means a longer travel time arriving in Miami southbound but a shorter time departing Miami northbound. So they'd *want* to back in heading northbound regardless. If they have a long southbound train, they may have to accept the extra time involved in backing in. This is tolerable.