Most of the crude oil reaching refineries in the Petroleum Administration for Defense District 1, which represents the East Coast market, is typically imported.
The Energy Information Administration, the Energy Department's analytical division, said crude oil volumes at PADD 1 refineries increased steadily during spring and summer 2013. At the same time, there's been a decline in net crude oil imports because of higher domestic production.
EIA said there's a growing supply of crude oil delivered to PADD 1 that can't be accounted for by production, imports or other forms of transport.
"Crude oil delivered via rail to East Coast refineries is likely contributing to the increase in unaccounted-for crude oil supply above historical levels," the administration said in a report published Wednesday.
EIA in its short-term market report, published last week, said U.S. crude oil production in August averaged 7.6 million barrels per day, the highest monthly level since 1989.
The Association of American Railroads said last week petroleum and petroleum products totaled 11,950 carloads, representing about 8.3 million barrels, for the week ending Sept. 7. That's a 4.8 percent increase from the same time last year.