A respirator is the last line of defense, and it's a lousy, limited one. A half mask or full mask, with a P100 particulate plus chemical, is a miserable thing to breathe through. If the user is not trained in its use and fit tested for a reliable seal, it's basically a fancy condom with a hole in it.

Emergency services and the railway will have a plan, in some detail, for such a contingency. They won't hand you the full plan because it contains personal data, names and phone numbers etc. But if you ask, they will usually give some guidance.

One of the biggest practical measures will involve having the local populace shelter in place. And that is the place to invest time and dollars -- making your residence and your shelter room as airtight as possible before an incident.