Update: Lead locomotive still in service, in New Jersey.
Shortly before 3 p.m. on Aug. 21, 1969, a crew of Penn Central workers had just gotten three locomotives ready to haul freight from the Dover Street yard in South Boston to a yard in Albany, NY. The fuel tanks were full and the engines were on to await the "hostler," the yard engineer who would drive the locomotives over to the freight area to connect them to the cars for their trip.
But then, even though the brakes were engaged, the three locomotives began to move on their own. The hitched locomotives started traveling around a loop in the yard - today known as the MBTA's Cabot Yard.
As they picked up speed, the locomotives moved onto a short spur line that ended at a chain-link fence along Frontage Road and the Southeast Expressway just before the Mass. Ave. exit. Only with no one to stop them - state investigators later concluded the crew that had been working on the diesels panicked and jumped off - the locomotives got up to a speed of about 30 m.p.h before the lead diesel smashed through the fence and just kept going, across Frontage Road and onto the northbound side of the highway.
The first engine made it most of the way across the northbound lanes before it stopped, gouging out what the Globe reported were three-foot-deep grooves in the road. The second diesel came to rest on Frontage Road; the third, its connection to the second broken by the jostling, behind it.
See this comment for more details on what happened.
Amazingly, nobody was hurt in the crash, although coming as it did a couple of hours before the afternoon rush hour, it created immense traffic jams on both sides - northbound because police were diverting traffic at Neponset Circle, southbound because everybody was rubbernecking - it's not every day you see diesel locomotives sitting on the highway.
The Globe reports that a Penn Central crew was able to pull the third engine back onto the tracks from which it had come, but that to get the other two off the road, they had to first bring in some temporary tracks and use cranes to lift the 125-ton vehicles onto them before they could be wheeled back into the yard. The Globe quoted a Penn Central official as saying this had never happened before.
The Boston Public Library's Brearley Collection, which has thousands of Boston news photos from the 1920s through the 1970s, has a number of photos of one of Boston's lesser known oddball disasters:
The South Bay incinerator, at left, is now the site of the Greater Boston Food Bank.
The crash came at a time of considerable ferment in the railroad industry. Penn Central itself had only been formed the year before, when the marginally profitable Pennsylvania Railroad and the equally modestly profitable New York Central merged (ultimately, the new railroad failed spectacularly). The second engine in the crash, in fact, still had New York Central logos emblazoned on it:
The Dover Street yard originally belonged to the New York, New Haven and Hartford Railroad, which Penn Central acquired only a few months before the crash. Four months after the crash, the MBTA board voted to spend $7 million to buy the 40-acre Dover Street yard, which it still owns today for Red Line and bus operations, but under the name Cabot Yard. Dover Street, meanwhile, was renamed East Berkeley Street.
Photos used under this Creative Commons license.