According to Kevin Morgan of ColoradoRailfan.com, the Longmont Turn, a BNSF local, rammed a tree stump placed on the tracks, presumably by vandals. The train was going near track speed of 40 MPH at 10:30 PM Monday evening and all four engines derailed as well as several freight cars in a massive dogpile. The lead unit, BNSF 8105, an SD-60M managed to remain upright.
The derailment happened at 95th Avenue and Wadsworth Parkway in Denver's northwest suburb of Westminster. Other Denver media, including 9news.com and the Denver Post (all news outlets), have picked up the story. Three crew members were aboard the local and one was taken to the hospital with minor injuries. Police still don't know who may have placed the stump or trunk on the tracks, only that it could have been much more serious had the train been carrying something other than building supplies.
Kevin has more pictures from this afternoon. Crews are still working to remove the debris to re-open the line. There is no current estimate on when the line will reopen.