PORTLAND – A steam engine that was being repaired for the Maine Narrow Gauge Railroad was damaged in an outdoor fire behind 58 Fore St. early Saturday, fire officials said.

A wooden frame over several steam engines that were being repaired caught fire around 4 a.m., said Bob Wassick, a deputy chief of the Portland Fire Department. The frame had been covered with plastic to provide workers with cover, Wassick said, but the plastic was blown off in a storm earlier this month.

Wassick said officials don’t know the cause of the fire, but an investigation is under way.

The steam engine was damaged by the heat of the fire, Wassick said, but most of the damage can probably be reapaired. He said parts of the engine, including the boiler, had already been removed.

The fire was brought under control within about a half hour, Wassick said, and there were no injuries.