UP Express train service between Union Station in downtown Toronto and Pearson Airport has been replaced by Express GO bus service until further notice.

The downage is due to “unexpected equipment maintenance,” the agency said in a release issued Saturday.

“We are providing an express bus service to Pearson Airport,” the statement reads. “Station Ambassadors are on site to assist.”

Express buses are currently picking up and dropping off riders on Station Street, and run every 20 mins in both directions.

On Friday, most of the fleet servicing the UP Express had been removed from operation after thermal cracks were discovered in the vehicle brake discs, according to the maintenance and operators’ union, leaving Metrolinx with only four train cars in service for the rail link connecting Pearson International Airport to downtown Toronto.

At the time, Metrolinx was adamant the four remaining trains had all passed safety inspections. By 10:30 a.m. on Saturday, they had also been pulled from service.

”Over the course of regular inspections this past weekend, our team discovered hairline cracks in the brake discs on some of our UP Express trains,” Metrolinx said in an emailed statement.

“In the interest of safety, we immediately removed the affected trains from service and proactively inspected the rest of our fleet.”

Brake discs are a form of braking system that have been in wide use in motor vehicles, trains, and aircraft for decades, since their invention in the 1890s.

Their life expectancy depends on the materials used, and are routinely serviced and replaced, said mechanical engineer Solomon Boakye-Yiadom, an assistant professor in the Lassonde School of Engineering at York University. The UP Express began operation in 2015.

The Toronto Transit Commission (TTC) said it is increasing service to Pearson International Airport for this weekend, specifically its 900 Airport Express bus route, “to provide customers with additional options.”

Buses will operate between Kipling Station and the airport every eight minutes or less, the TTC said.

The 900 Airport Express bus only stops at Kipling Station, Dundas Street West and Acorn Avenue, Dundas Street West and East Mall Crescent, Jetliner Road at Airport Road, Terminal 1 and Terminal 3.

In addition, the TTC said it is also increasing service on the 52A Lawrence West with buses travelling between the airport and Lawrence West subway station.

“These service increases will be in place until end-of-day on Sun. Feb 12. The TTC will continue to monitor passenger volume to determine if airport service will be increased into next week,” the transit agency said in a statement.

With files from Andrew Brennan