Here is the story it appears in the SamTrans.org website.
"Luggage Policy on Route KX
SamTrans is unable to allow excess luggage to be carried on Route KX between San Francisco and the San Francisco International Airport in either direction.
SamTrans Carry-on Policy States:
"If a passenger can carry the item(s) on his/her lap or store it under or next to him/her on a seat unneeded and not hurt anyone else, and not interfere with others, the item is probably safe to transport." The final decision will be made by the bus operator, who is responsible for the safe transport of everyone on the vehicle.
Alternatives & Information:
If you are unable to ride Route KX, Route 292 can take you to The City or the airport.
SamTrans wants your trip to be as pleasant as possible. If you have any routing or fare questions, please call 1-800-660-4287.
Since SamTrans receives some federal money to help fund its operation, a private bus operator cited "unfair competition." Under an agreement with the Federal Transit Administration, SamTrans agreed not to transport passengers with luggage between The City and the airport on its express route.If the item fits into the following definition, SamTrans will gladly allow the passenger to board with the item. "