Pass Card

In Fallout 3, s are items that are randomly dropped by ghouls or centaurs. They may also be found on trogs if The Pitt is installed.

There is no place where a pass card is guaranteed to be found. A ghoul has a 1 in 80 chance of dropping a pass of either color; for a centaur the chance is about 1 in 170. Trogs have an even smaller chance - about 1 in 240. The cards have no purpose: they are not accepted by Protectrons in lieu of metro tickets.

Trivia
The presence of these items in Fallout 3 may be a nod to the preceding games in the series as Fallout and Fallout 2 featured Red, Blue, and Yellow Pass Keys which were used to open corresponding doors. Also this could be a reference to Doom, where you had you had to find tons of red and blue keys that looked like cards in game.