Sergeant Mike Schaller, spokesman for the El Paso County Sheriff's Office, said a crew on a southbound train noticed a vapor trail emitting from a northbound Burlington Northern Santa Fe Railway rail car early Wednesday morning.
"It was not a rapid leak, but when the liquid dripped out and hit the ground, it turned into a vapor," Schaller said.
Emergency crews unhitched the leaking car from the train because another car in the train was carrying a chemical that could cause problems if it came in contact with the acid, he said.
The freight train originated in Kansas and was en route to Denver, Lena Kent, spokeswoman for Burlington Santa Fe, told Reuters.
Railroad hazard materials crews applied wax on the car, which slowed the leak and applied a magnetic patch to seal the tanker, she said.
Schaller said police went door-to-door notifying residents of a subdivision adjacent to the railroad tracks to evacuate as a precaution, and the local school district decided to cancel classes until further notice.
No injuries have been reported, he said, but residents have been warned that they may have to be out of their homes until Friday as hazardous material crews offload the acid to another container.
Kent said a crew from Fort Worth, Texas that specializes in transferring hazardous material was summoned to the site.
The incident has backed up the company's rail traffic, with 15 trains "holding" until the track is cleared, Kent said
The car was carrying 25,000 gallons of hydrochloric acid, a poisonous, colorless solution that has numerous industrial uses, according to the U.S. Environmental Protection Agency.
"Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes," the agency says on its web site.
Monument is a town of 3,230 residents 55 miles south of Denver.