DOON, Iowa — Cleanup efforts began Friday at the site of a derailed BNSF Railway freight train that spilled thousands of gallons of crude oil into a swollen river in southern Lyon County in far northwest Iowa. The derailment forced the evacuation of residents from several neighboring farms and sent oil downstream to Rock Valley, where hundreds of workers and volunteers spent much of Friday morning sandbagging and building berms to protect homes on the city’s west side from the flooded Rock River.Though investigators have not yet identified the cause of the accident, local authorities believe floodwaters from the nearby Little Rock River caused the ground to weaken beneath the tracks.