I honestly get the feeling that this service could end up being an entirely new MTA branch, similar to SIR, but independent of the subway. It's too long to be a Light rail or bus rapid transit line, and too short to be an MNRR/LIRR line unless it offshoots from existing trackage from those services that are still active. It would do best in my option as either a full on subway line (with some major upgrades and expansions) or as i mentioned above, an SIR type service.