Spotted this downtown recently:
6019 is the closest thing the MBTA has to a minibus. I have only spotted it in revenue service once; at Ashmont Station assigned to Route 798 (Mattapan Trolley Shuttle via Eliot Street). It does not have a farebox (last time I checked), so who knows how the passengers paid back then. Its primary function is to serve as a shuttle for transit officials on excursions around the system.
Wouldn't it be interesting if the T had a small fleet of these?