For the record, SEPTA (Philadelphia's multi-modal mass transit system) Police have probably the best interop arrangement in the entire Philly region. In fact, one of the Lt's at SEPTA PD has been a major driving force in getting the Philly metro area to think about interoperability.
SEPTA maintains a RIOS set-up that can be activated locally or remotely and can link numerous disparate systems together, if needed.
The biggest stumbling block, as I see it, is getting the people sitting at the consoles and the people holding mics in their hands to understand what they have available and what it can and cannot do.
SEPTA Transit Police operate on a conventional analog repeater in the 500MHz band. Philly PD J Band (flash announcements, admin traffic, etc) is continuously broadcast on SEPTA's dispatch channel since the majority of their work is in Philly.