Yes, under Department of Transportation (DOT) Americans with Disabilities Act (ADA) regulations at 49 C.F.R. Section 37.121(a), “each public entity operating a fixed route system shall provide paratransit or other special service to individuals with disabilities that is comparable to the level of service provided to individuals without disabilities who use the fixed route system.” The regulation does not specify how this paratransit service is to be provided—whether by vehicles from its own fleet, by vehicles from a subrecipient, or by vehicles from a for-profit third-party contractor. The regulation only requires complementary paratransit services to be provided, leaving the details to the local planning process. The fixed route operator, however, is ultimately responsible for ensuring a contractor meets all applicable ADA requirements as explained in Section 37.23. You may view the cited regulation here.