Only one TJs in Miami Dade county, in Pinecrest. By coincidence the Metrorail line from the airport area terminates across the street.

But .....

The train journey itself is about one hour each way, and you need more time each way to take the people mover to and from the airport to the station (train doesn't come into the airport proper).

Then you have to clear security. 5 hours is not enough time from a realistic standpoint.