Updated with news about another trolley getting snagged tonight.
A mechanism at the mouth of the Riverside Line's Fenway tunnel that is supposed to help protect the tunnel from flooding is now damaging some pantographs that power trolleys as they move between the underground and surface-level tracks.
The Riverside Line has been knocked out of service twice just today after the mechanism wrecked the pantographs - which connect the trolleys to the overhead power lines.
At least three trolleys today suffered damage severe enough that the T shut down service completely between Fenway and Reservoir this morning and started running buses instead, a source said.
Normal service resumed around 5 p.m., but another trolley got damaged tonight and the T is now scrambling to find buses to run as shuttles.
The mechanism is designed to close two large steel doors at the tunnel's "portal" in the event of potential flooding, to prevent a recurrence of a 1996 flood that shut Kenmore for a week and sent water into the Green Line tunnel as far away as Arlington Street.
The work is in addition to an ongoing project by the Army Corps of Engineers to reduce flooding along the Muddy River - the most visible part is the now "daylighted" section of the river in front of the Landmark Center, formerly an inadequately sized tunnel under a parking lot.
Although the $22-million MBTA flood-gate project was finished in late, 2020, the T is now trying to figure out what exactly is sometimes denting the pantographs that reach up from trolleys to the overhead power lines. Drivers were already supposed to slow down as they passed through the portal.
In today's incident, the drivers of two damaged outbound trolleys were able to get to Beaconsfield, while a third, inbound trolley made it to Fenway. The T was able to get all three to Riverside for repairs while workers tended to the wires at the portal itself.