I think I'd get off at the very next stop and wait for the next train.

A bag probably wouldn't have helped her as she most likely didn't have much notice before the "big moment". And if she'd felt it coming for a while most likely wouldn't have asked if anyone nearby had a bag.

A mask might have a helped a little, but I'd guess there isn't much of an aerosol-effect. The main objective is to open windows and get off the the train ASAP.

One of the train operators needs to be told as soon as possible too as this is a biohazard and fall hazard too.