Per an article in the Boston Globe, Massport will conduct a feasibility study on building a train or monorail to connect the terminals and the rental car center.

Boston Globe article

In my opinion, it seems like an worthwhile thing to study but the billion-dollar price estimate certainly raises concerns. Perhaps building a third level of roadway would be a more viable option. You could have it only be for buses, which would include shuttle buses as well as hotel shuttles and Logan Express, etc.