Metro is sidelining its 94 Orion VI diesel buses after two caught fire in the past week, transit officials said Tuesday.

In both cases, there were no passengers aboard and the drivers were not injured, according to Metro spokesman Dan Stessel.

The most recent fire happened Tuesday morning in the rear of a bus that a mechanic was driving along Route 50 in Maryland in a test run.

The second fire occurred about 8 p.m. Thursday as a bus was headed westbound on Interstate 66 near Nutley Street. That fire also started in the rear of the bus, according to Stessel.

“Given the fact we’ve had two fires involving these buses in the span of a week, it is a prudent step to take while we investigate the underlying cause and determine the next steps,” Stessel said.

Metro officials said in a statement that the buses will remain out of service “until further notice.”

The Orion VI vehicles are a small part of Metro’s fleet of 1,492 buses. Metro said it bought the buses in 2000 for $282,000 each.

Silke Walters, a spokeswoman for Daimler Buses North America, which owns Orion, said in an e-mail Tuesday: “We do not comment on any customer issues.”

Since the fires, Metro officials said they have hired an “outside fire protection engineer” to help the transit authority’s safety department investigate the incidents.

Stessel said other buses that are usually in reserve will be used to replace the 94 buses used on routes in Prince George’s, Fairfax and Arlington counties, Northeast Washington, Alexandria and Garrett Park.

“Customers should see little to no impact,” he said.

The latest service problems come after Metro said 47 of its new hybrid buses are being repaired because they can develop shorts in their battery systems.

Last week, federal safety officials said they are investigating fires that have destroyed five of Montgomery County’s Ride On buses since 2009. Officials have said those vehicles, made by Navistar, have problems with electrical panels and parking brakes that caused the fires.