Why are knives more effective than train tracks?

Interesting question. At least in terms of cartoon violence, the damsels or other goodie-goodies usually escape from being tied to the train tracks. The hero always comes along at the last minute. If a bad guy used a knife instead, then even if the hero came along, the damage would already be done. Not that I am advocating for bad guys, at all... Good guys are a lot cooler. But that is a way that I can imagine a knife being more effective than train tracks.