That entrance to the BART/Muni station at the foot of Grove Street, in front of Burger King, is a thing of the past, and BART will also be closing the corresponding entrance on the opposite side of Market Street at Eighth, which connects to the station itself via tunnel. BART announced the closures in a release Tuesday, explaining that the real estate being used by the entrances and tunnel is needed for a new power substation that will increase BART's hourly capacity through the Transbay Tube at peak hours.
From the release:
Looking towards the near future, BART needs to install a new traction power substation at Civic Center as part of our Measure RR rebuilding efforts and strategies to increase capacity into and out of the city. New substations, along with upgrades to our train control system and a new fleet of rail cars will allow BART to increase peak capacity by 25% through the Transbay Tube. We currently run 24 trains per hour in each direction through the Transbay Tube, these planned improvements will enable us to increase to 30 trains per hour.
Hoodline notes that at least one of their readers was alarmed by the news because it means that there will be two fewer exits from the busy station in case of fire. But the Grove Street entrance has actually been closed since December due to a collapsed sewage pipe beneath the escalators, and many who are familiar with these entrances know that that weird tunnel wasn't always the most pleasant walk anyway.
Per the release, "While the station will likely lose these two entrances, there are others still available that are located much closer to the platform, fare gates, and ticket vending machines and riders will gain the benefits of more frequent and more reliable service."
Indeed, on the other side of Eighth Street, at Market (in front of the Orpheum Theater and Trinity Place on the opposite side) there are the two main entrances to the station, situated near the end of the Muni platform where outbound trains stop. And then there is the large escalator in UN Plaza, as well as two other entrances on either side of Market at Seventh Street.
BART isn't saying for certain that the entrances will be going away, but that seems to be almost certainly the case.
In the short term, look for the wooden structure over the Grove Street entrance to disappear and be replaced with a flat metal plate over the entrance.