plot I have no use for again, but this time it's terrorist:
need:
city where it's possible to take a metro out of city and back, it's possible to pay for metro with cash, and people aren't searched before going on the metro, and the metro trashbin are analogue.
skill with makeup
androynous appearance
self or ally needs to be able to make bombs and poison.
time tables for metro. specifically, when and where there will be many people waiting for the metro during the evening rush.
private place outside town.
leave town with:
enough time to go to town, spend some time there (buy a book and cocoa and read a bit?), then leave on the metro before the fullest one.
cash for metro trips and to spend some time.
at least one improvised explosive with poisonous fragments, made inside a soda-can or other reasonably sized trash. remote or timed detonation. I've heard the effective range of a hand-grenade is further than it can be thrown, which would indicate this scale can do reasonable damage if it's poisonous.
your own pre-paid card.
wearing your normal clothes
no electronics.
a disguise as a member of the opposite sex in a backpack. casual clothes. for a man: tights, oversized (past groin) woman's hoodie, padded bra, fabric shoes. doesn't draw attention, but they'll be looking for the wrong gender based on security-tapes.
don disguise, hide backpack with everything but bomb and cash and disguise in private place.
buy two-way ticket.
go to place you want to bomb.
leave station and spend some time.
activate explosive, timed for busiest time.
drop explosive in a trashbin at the busiest stop.
take metro out of town.
just as next metro arrives: boom!
put normal clothes back on and clean off makeup. throw disguise into a volcano.
take metro or maybe bus because of the terrorism using your own cards. you were just out of town.
you probably just blew up and poisoned wherever the analogue metro janitors dump the trash. now everyone has to recycle everything.