Box in the railcar

 is a stash featured in S.T.A.L.K.E.R.: Lost Alpha.

"Crap, this shit is weighing me down. There's so many of them! I gotta leave this."

- PDA description

Overview
Contrary to the name of the stash, the car is not rail, and the “box” is a part of some kind duct.

Trivia

 * All stash coordinates in Pripyat City require the player to actually be on that level.