Irrational Games has released a developer diary that outlines some of the history behind the "Sky-Lines" in BioShock Infinite.

As explained by Ken Levine, the rail lines that connect sections of Columbia--the game's set-piece floating city--were originally intended to be used for transport of goods. Evidently, a bunch of pesky kids realized they could use the Sky-Line for personal transport and, as the city began to self-destruct, a new method of travel was born.

It's a great, subtle narrative detail for one mechanic in the upcoming game. As noted in our E3 2011 preview, we have high hopes for BioShock Infinite, which is set to launch on the PC, Xbox 360, and PS3 in 2012.