49 C.F.R. § 37.35

(a) Transportation service provided by bus or other vehicle by an intercity commuter or rail operator, as an extension of or supplement to its rail service, and which connects an intercity rail station and limited other points, is subject to the requirements of this part for fixed route commuter bus service operated by a public entity.

(b) Dedicated bus service to commuter rail systems, with through ticketing arrangements and which is available only to users of the commuter rail system, is subject to the requirements of this part for fixed route commuter bus service operated by a public entity.