You missed the " B Train " Bryan :B but your correct.
State's can also have different licensing requirement. Here a doozy from Calif.
DRIVER LICENSING for DOUBLE TRAILERS - CALIFORNIA RESIDENTS
12804.9. (b) ..., any applicant for a driver's license shall be required to submit to an examination appropriate to the type of motor vehicle or combination of vehicles the applicant desires a license to drive:
(1) Class A includes the following:
(A) Any combination of vehicles, if any vehicle being towed has a gross vehicle weight rating of more than 10,000 pounds.
(B) Any vehicle towing more than one vehicle.
(C) ...
15278. (a) A driver is required to obtain an endorsement issued by the department to operate any commercial motor vehicle that is any of the following: (1) A double trailer. (2) ...