Hmm... We haven't actually seen Canaan to be excessively revenge driven (like so many shounen characters). She hasn't been going out of her way to kill Alphard (though she hates her surely) but has prioritised being with Maria in my opinion. In that sense I tend to think a threat of Maria dying might indeed be more powerful than her actually dying. It's a good theory the train car might not explode at all. Although an empty threat wouldn't really be conducive in the long run.