I can see this working out in very limited cases:
At stations w/parking, outside city limits:
A vendor will sell packaged food that must
be heated up -- like TV dinners, ready-to-cook family dinners, ready-to-bake pizzas, whatnot. Stuff that you don't take on the subway, because you've just gotten off the subway and face having to *cook*. But why not have a small pot roast, packaged and chilled, ready to cook so all you do is heat up the oven, slip it in, wait 30 minutes, dinner?
The same goes for DVD rental and video game rental. You're going home for the weekend, so why not pick something up and return it on the way back in? RedBox or DVD Express!
Lotto tickets... is a mixed bag. I see them more effective on the outskirts than in the city (inside the beltway). Compare New Carrolton to Union Station -- I think New Carrolton has it's MD Lotto vendor there, while Union Station took out all theirs via attrition. New Carrolton can use a (newer) MD Lottery ticket machine (LTM).
At inside the beltway stations:
I can see the Photo booths here, as you're outside Verizon Center after that big major concert with your friends, you're slightly drunk, and you can't hold your phone worth a damn. Pile into a booth, get a few shots!
Health & Beauty, Electronics (Best Buy Express), and charging stations or charger rental (a la Fuel-Rod
at airports) all belong here, for numerous reasons. The overlap of customer types (tourists, family folk, businessmen, etc) would permit a lot of sales.
The questionable one is:
Package/mail pickup/delivery lockers. Post office, maybe, but depends on the hours set. Amazon locker? I can see it on the outer edges but not inside the beltway.