I don't know if you've ever ridden the subway, but coming into contact with someone's bare skin, even broken skin, is not unheard of in a crowded subway. So it's plausible that your cat scratched arm could come in contact with some infected person's sweaty arm as you try to make your way through a crowded subway car.

It's certainly plausible that someone with an early stage infection may take the subway to go to a doctor. I've seen plenty of obviously sick people on the train (and I keep my distance, not because I think they have Ebola, but because I don't want to catch whatever illness they do have)