Excerpted from the "ADA Title III Regulation 28 CFR Part 36"
https://archive.ada.gov/regs2010/titleIII_2010/titleIII_2010_regulations.htm
Sec.36.310 Transportation provided by public accommodations.
(a) General.
(1) A public accommodation that provides transportation services, but that is not primarily engaged in the business of transporting people, is subject to the general and specific provisions in subparts B, C, and D of this part for its transportation operations, except as provided in this section.
(2) Examples. Transportation services subject to this section include, but are not limited to, shuttle services
operated between transportation terminals and places of public accommodation, customer shuttle bus services operated by private companies and shopping centers, student transportation systems, and transportation provided within recreational facilities such as stadiums, zoos, amusement parks, and ski resorts.
(b) Barrier removal. A public accommodation subject to this section shall remove transportation barriers in existing vehicles and rail passenger cars used for transporting individuals (not including barriers that can only be removed through the retrofitting of vehicles or rail passenger cars by the installation of a hydraulic or other lift) where such removal is readily achievable.
(c) Requirements for vehicles and systems. A public accommodation subject to this section shall comply with the requirements pertaining to vehicles and transportation systems in the regulations issued by the Secretary of Transportation pursuant to section 306 of the Act.