Metro officials explained Thursday how part of a six-car train derailed last month in the Brentwood rail yard and tried to assure their board of directors that a similar incident was unlikely to happen on the main line tracks that carry passengers.

Rob Troup, the head of Metro’s rail operations, used a piece of a train’s wheel equipment to explain how several factors led to the incident, which had no reported injuries.

He cited a “stiff” piece in a part, combined with the train’s slow rate of speed on a curved part of track. Plus, the ballast — or crushed stone that lays in the track bed — was slightly uneven. All of that, he said, helped lead to the derailment.

Troup said there are different standards for ballast and other measurements of the train for the main line tracks, where passengers ride, so it was not likely that such factors would combine and lead to a derailment.

Metro General Manager Richard Sarles described the incident in more detail after committee meetings.

“You had new rail, new wheels and a dry day,” Sarles said. “Because of the dry condition and a new wheel, it didn’t slip up against the rail as it normally would, and because of the ballast. It climbed up and over the rail.”

He also said it was a “combination of the track moving down and it being so dry and there being new wheels” that led to the derailment.

On the main tracks, he said, “you maintain tighter tolerances and that wouldn’t happen.”