There is an interesting report available on the web that is "A Weekly Report of North American Rail Freight Traffic by Major Railroad and Commodity." It is currently showing data for week 39 of 2008. Take a look.
Here is an example of the information available on this web site. This graph shows Total Traffic for 2007-2008 vs. 2006-2007. There are more graphs and tables on the web site showing data by railroad and by commodity.
Note: This web site has new tables and graphs every Thursday. So, if I am late or miss a week with this blog posting, you can check for new data on Thursdays.