The Goblin Fire in the San Juan National Forest north of Durango spread to an estimated 475 acres on Thursday.
As the fire advances in steep, rocky terrain that prevents efforts at containment, firefighters are building a line around two cabins and two outbuildings about ¾ of a mile down the Animas River from the fire.
The fire covered 110 acres Tuesday and 350 acres Wednesday.
Fire managers think the fire was sparked Saturday by an ember from a Durango & Silverton Narrow Gauge Railroad locomotive.
Smoke from the fire is visible from the north Animas Valley.
Four U.S. Forest Service firefighters are keeping the fire away from the Animas River and the railroad track, agency spokeswoman Ann Bond said.
The railroad has seven firefighters on standby, Bond said.
The fire has spread upslope, and it is now burning into the West Needles portion of the Weminuche Wilderness, Bond said.
An area further upslope in the wilderness that burned in 2002 is expected to act as a fire break, Bond said.
Steep, rocky terrain has limited firefighting activity. Firefighters are containing the fire where feasible and safe, Bond said.
The low-intensity fire is following the same pattern, continburning dead needles and other fuel on the ground. Conifers are not catching fire but Gambel oak and junipers are torching in some areas.