I don't have any real suggestion, but I do think it should probably be some kind of three or four letter acronym. For example, in SanFran it's BART. In Portland it's MAX.
SAX? Seattle Area Express. Like a saxophone.
SART? Seattle Area Rapid Transit. Like that French philosopher, Sartre.
I don't care much for either of those. We've got to come up with something more original.