If it requires special equipment be installed on BART, then I don't see how it is a right. If you can use a cellphone without BART installing and maintaining special equipment, then they have a better case.
People can use cellphones on the highway and in the city with no special equipment.
It is not a constitutional right as I see it, because when the Constitution was written, there were no telephones. A cellphone call would have seemed more like magic than technology to the Founding Fathers, I am sure, perhaps even witchcraft.
Cellphones would have been quite useful in preventing slaves from running away, as would photography and fingerprinting.