Source: U.S. Bureau of Transportation Statistics  

Release: Transportation Services Index and Seasonally-Adjusted Transportation Data  

Units:  Containers and Trailers, Not Seasonally Adjusted

Frequency:  Monthly

Notes:

Monthly rail carloads and intermodals are estimated by dividing the weekly sum by 7 (7 days in a week) and then summing for the number of days in the month (31 days for May, 30 days for June, etc.).

This data is collected by the Association of American Railroads.