It's AAR rail waste carloads, a coincident indicator that has a pretty strong correlation with U.S. GDP.
And based on the most recent readings, it's telling us that GDP growth is slumping in Q3.  Unfortunately, we won't start getting Q3 GDP readings until October.
Bloomberg BRIEF economist Michael McDonough follows the indicator closely.  Here's his chart:
garbage indicator

Here's another chart from McDonough taking a looking at the monthly measure on a historical basis:
garbage indicator