Ok, let's go with he did sneeze or cough on the subway...
So? Ebola is damn near impossible to get the way you described, in fact there hasn't been one documented case of anyone catching it by touching a surface that way ever.
The CDC has even cited a study where surfaces, everything from bed rails to spit bowls, etc. in an active ebola ward were tested for the virus, and the evidence just isn't there.
The chance he was up and around before the onset of symptoms is slim to none, considering
a. he was taking his temperature twice a day since returning to the U.S.
and
b. studies show the ebola virus doesn't show up in saliva until/unless the virus has progressed significantly in you. You'd have to be incredibly sick for your saliva and such to be the issue, unlike feces, vomit, blood.
Meaning he could've done worse on that subway than cough, and all passengers would've been a-ok.