A TRUCK ON FRIDAY morning entered the Silver Line underground transitway that runs to South Station and caused damage to the overhead wires, which had not been fully repaired as of Saturday evening.

Lisa Battiston, a spokeswoman for the MBTA, issued a statement saying the truck entered the transitway at approximately 9:30 a.m. by mistake. Until the repairs are completed, she said, Silver Line service will operate at street level.

The transitway goes underground at D Street and continues to South Station after making stops at World Trade Center and the federal courthouse. In the transitway, buses operate on electricity. Signs at the entrance say the transitway is limited to buses only.

The entrance at D Street is restricted to buses, and has a crossing barrier that prevents other vehicles from entering. Battiston’s statement did not explain how a truck could have entered the tunnel.

Jim Aloisi, a member of the TransitMatters board and a former state secretary of transportation, said the truck incident raises serious questions about the adequacy of security on the transitway. He said an unauthorized vehicle should not be allowed to enter the transitway and be allowed to travel underground in the Seaport District and end up underneath South Station.

In October 2013, a driver was arrested after driving into the transitway and charged with drunk driving.