Even though this thread seems like a wind up, with the suggestion that while someone is right in front of you with a knife after stabbing loads of people, you call a ticket inspector or wonder if there's an emergency alarm, I believe that one thing you can do is pick up a seat base and use that as a shield.
It's what I've seen trainee RPIs being told, as the seat bases come up quite easily (well, actually the seat backs do too, as I noticed when I knocked one accidentally and it came off!) on certain stock. In this case, it was National Express telling RPIs on a 317.
The metal/wood that holds the seat base in place even acts a makeshift handle, giving you a rather cheesy but effective shield. If enough people did this, I am sure you'd be able to force the offender back. Okay, maybe the base wouldn't be strong enough for a massive knife, sword, axe or sub-machine gun, but it's better than your bare hands or arms!
Personally, I'd be texting Chuck Norris though as it would be far more effective.