One of the Massachusetts Bay Transportation Authority’s new Orange Line trains derailed in a train yard mishap Monday, according to the MBTA.

Spokesman Joe Pesaturo said the train wasn’t in service, and had no passengers aboard.

The train was on its way back to the rail yard at noon when the last car of the six-car train popped off the rails, he said.

The train was moving slowly as it went over a track switch when the car jumped the track.

The exact cause of the derailment is under investigation, Pesaturo said, adding that an immediate inspection “no significant damage” to the train.

Pesaturo noted that the switch in question is scheduled to be replaced as part of the $102 million overhaul of the Wellington Rail Yard.

The car is one of the first of 152 new Orange Line trains that will be rolled out over the next couple of years as the T entirely replaces the aging Orange Line fleet. The first six-car train began carrying passengers in August — though the first two trains were temporarily taken off the rails when a door popped open while one of the trains was in motion.

The T has hired an outside firm to take a look at train safety protocol after multiple high-profile derailments earlier this year, including a Red Line train that jumped the tracks during rush hour and demolished a signal station over the summer, leading to months of delays. The Green Line and Commuter Rail also saw multiple trains depart from the tracks.