A stretch of defective track caused a Green Line trolley to derail near Packard’s Corner in Allston, MBTA General Manager Philip Eng said.

Roughly 30 passengers were aboard when the derailment occurred around 1:40 p.m. Monday, and were able to safely exit the westbound train. There were no injuries, T spokesperson Joe Pesaturo said.

Buses replaced B branch service between Kenmore and Washington Street while T personnel worked to re-rail the trolley car. Eng said the alternative service would continue through rush hour, and possibly into the evening, should crews decide to do more extensive track repairs.

“Our rail track team is out here assessing any rail repairs that need to be done, and right now we believe what contributed to it was the track condition,” Eng told reporters late Monday afternoon. “Preliminarily, the distance between the rails was not where it should have been so we’re assessing how that happened.”

Eng said the center truck, or center wheels of a two-car set on the trolley came off the tracks. He said the derailment is still being investigated, to determine if anything beyond poor track conditions was a contributing factor.

The safety failure was reported to both of the MBTA’s oversight agencies, the Department of Public Utilities and the Federal Transit Administration.

“Obviously we have a lot of work to do,” Eng said. “The one thing that I will say is we did have a speed restriction in place here. That’s why it was so low-speed. That’s why no one was injured. And that just shows we have a lot more to do.”

The derailment comes ahead of track rehabilitation work that was already planned for the area this summer, Eng said.

One of the T’s “on-call contractors” will complete track repairs over the course of 12 days on the B branch of the Green Line, from Babcock Street to Packard’s Corner stations, Pesaturo said.

“We have a lot of competing priorities as far as where we need to tackle,” Eng said. “This is one of those that we’re going to look to accelerate.”

To that end, spot repairs may continue Tuesday or Wednesday, Eng said.

Riders encountered delays following the derailment on Monday. The Route 57 bus was temporarily free between Kenmore and Washington, for those seeking an alternative.

The T initially communicated the incident as a “disabled train near Packard’s Corner” on social media, but later tweeted that the delays were caused by a trolley that had, in fact, derailed.