IMG_3477 by Micah Sittig (cc by)
In the wake of the NYC subway tragedy, MetaFilter members wondered why we couldn't have sealed subway platforms with trains stopping only at doors (like those pictured above, called platform edge doors - PEDs). Users sevenyearlurk breaks down the immense cost of retrofitting lines and garius explains the difficulties of getting them to work correctly:
Just to provide some kind of summary on the whole "platform doors" thing, it's always worth remembering that, when it comes to Metro systems, it's generally pretty easy (and cost effective) to do stuff as long as its done or planned for when the system is built. After that though you're pretty screwed.