[LaGrange]

Metro is a very specific case, and that line in specific - you have a limited variety of vehicles and fully enclosed platforms. Oh, and as far as I can remember, the specific metro line is fully automated.

In practice the incidents are rare (even in case of metros, it's something you read in news papers every time it happens), and the few attempts to implement them were generally underwhelming. And, if anything, the risk of a person getting between them and the vehicle means you're just swapping risks around.

So yeah, they're costly, of dubious use, they're yet another part that can break, and a general eyesore. And, if you want dense networks (and you do, trust me on that), then increasing the amount of space required for a stop is also a Bad Thing.