With monitoring ongoing, responders to last week's oil-train derailment in West Virginia said they're busy removing an oily mixture from containment trenches.

Nearly half of the 28 cars that derailed from a CSX line last week near Mt. Carbon, W. Va., were carrying crude oil from North Dakota. If filled, that would represent about 8,000 barrels of oil on board.

A unified command set up by rail company CSX, local, state and federal authorities said crews pulled about 3,950 barrels of oil directly from the tank cars as of Monday, up about 300 barrels from the previous day. An additional 850 barrels of oily-water mixture were removed from containment trenches dug along the Kanawha River.

The U.S. Environmental Protection Agency said a "small amount" of oil had spilled into the river.

As of Monday, the unified command said four of the derailed cars still need removal. Once those operations are completed, crews will remove the impacted soil in preparation for new tracks.

The increase in U.S. oil production is more than the existing network of pipelines can handle, leaving energy companies to rely on rail as a supplementary transit method. A federal report found oil-by-rail volumes increased from 10,800 carloads in 2009 to 400,000 in 2013.

The string of derailments, including a deadly 2013 incident in Lac-Megantic, Quebec, has raised safety concerns about transporting oil by rail. Most derailments involved rail cars designated DOT-111, though last week's cars were of the federal CPC-1232 classification.

A briefing Monday from the Pipeline and Hazardous Materials Safety Administration found CPC-1232 were "marginally" better than older cars.