Pass Card

In Fallout 3, s are items that are randomly encountered.

Locations:
Pass cards are located in random places throughout the Capital Wasteland. They are dropped by ghouls or centaurs, and may also be found on trogs if The Pitt is installed.

A ghoul has 1 chance in 80 of dropping a pass card of either color, A centaur has a chance of about 1 in 170, & trogs have an even smaller chance of about 1 in 240.

Notes:
The cards seem to have no purpose so far. (they are not accepted by Protectrons in lieu of metro tickets!)

They also appear to just be the pass cards that let riders on the trains, both red and blue. Obviously expired by now, they serve no purpose.