Warren Buffett's only piece of economic data to keep up with the health of the U.S. economy

Warren Buffett was asked once on CNBC that if he were on a secluded island and he was allowed only once piece of economic data to keep up with the health of the U.S. economy, what would it be? He responded that he would want to know the level of U.S. rail car loadings. The higher the activity level the better the economy.
Throughout 2006 to late Q3/08 rail car loadings were in the 320 – 345K level per week. In late 2008 this
fell sharply as the economy weakened during the financial crises. With the financial crises over, the
level of car loadings has not picked up. The January 2010 data shows a decline of 0.7% to 264K/week
from a year ago and down 18% from 2008’s average weekly number of 321K/week.