I travelled to the emergency exit.
Looks like it's either bolted shut, extremely heavy, or there's some convoluted way to open it.
It must be possible to get in because rumor has it that some people have gotten it and taken pictures, but never posted them online because of how strict DART is about trespassing.
Album of me trying to open the trapdoor: Knox henderson station emergency exit https://imgur.com/gallery/Y2GzZ
There might be a hidden way to open it from that small flap, but I can't play around because it's eight next to a busy highway where some goody two shoes driving past might call the cops. The trapdoor seems bolted shut. I'd have to revisit this at night when there are few cars.
It's possible that it can only be opened from underneath