Officials with the San Francisco Municipal Transportation Agency are in the process of replacing 61 Muni trolleys.
The move, however, was not prompted by two separate electric carriages bursting into flames on Tuesday and Thursday last week, Muni officials say.
The trolleys have undergone significant wear and tear in their 15-year life span and the swap is part of a normal replacement schedule, they stressed.
According to Muni officials, the problem lies in a small piece – it looks like a bolt – that connects the pole on the top of the trolley to a metal shoe that runs up against the 600 volt power lines suspended above city streets.
Last week, those connectors overheated and sparked fires on the tops of two buses, officials said.
In both cases, the Muni carriages were climbing steep hills – the first at Ocean Avenue in front of City College and the second, near Union and Mason streets. Both drives stopped the trolleys and disconnected the power poles from the electrical lines, preventing the fire from entering the coaches, according to officials.
The affected Muni carriages were taken out service and officials began inspecting the roughly 300 coaches that traverse San Francisco. They found that about 50 need to be retrofitted with a new connector.
Per Muni’s Director of Transit John Haley, the trolleys are 15 years old and near the end of their service life. Wear and tear is a "fair way to describe" the cause of the fires, he said.
"These coaches on average have run 500,000 miles," Haley said, adding that the coaches should ideally be taken off the streets after about 300,000 miles.
Muni officials hope to replace nearly their entire fleet in the coming two years.