I think of it as anything anyone can buy a ticket on to get someplace. As a matter of fact, Amex uses the term common carrier in their terms, but it also means a bus or train, in addition to air, doesn't it? I recall wondering (I think in this very thread) why a trip that takes place in your own vehicle wouldn't be covered, and I think that was the only thing not considered a common carrier.

So a strike by any train, bus or air employees would all have the same effect of delaying someone.