The Energy Information Administration, the department's analytical arm, said it doesn't track domestic movements of crude oil by rail. It found, however, there is a growing supply of crude oil on the West Coast that can't be accounted for by production, imports or other means.
"This unaccounted-for crude is likely crude oil delivered via railroad to refineries on the West Coast," the administration said Wednesday.
There are no major pipelines feeding the refinery market on the West Coast. The EIA said oil products for the West Coast are transferred normally by maritime deliveries through the Panama Canal.
The Energy Department last week said crude oil production in July increased to 7.5 million barrels per day, the highest level in more than 20 years. In response, some energy companies have turned to rail because higher production levels strain existing pipeline capacity.
The Association of American Railroads said rail traffic was up 2.5 percent in July when compared with last year. A weekly average of 243,725 carloads in July was the highest average for any July in history.