Among the 21 categories of items shipped by rail, none have a tighter correlation to GDP than waste.

According to a 2010 piece on Bloomberg, economists Michael McDonough and Carl Riccadonna note that waste has an 82 percent correlation to US economic growth.

This should be pretty intuitive. The more you produce, the more you throw out.

McDonough, a Bloomberg BRIEF economist, tweeted out an update on the indicator.

And frankly, it stinks. Waste carloads are way down.