BART will permanently close an entrance to its Civic Center station at 8th and Market streets Thursday, shutting off a portal to the Hotel Whitcomb and several theaters in San Francisco’s struggling mid-Market arts district.

The move is necessary to make room for a new traction power substation, which will occupy space underground formerly filled with escalators and tiled corridors — and seemingly omnipresent injection drug users.

Another entrance at Grove Street in front of Burger King was shuttered in 2016, leaving six exits and entryways between the transit hub and the corridor flanked by government edifices, museums and performance halls.

Merchants and managers of these institutions met with BART throughout the year, fearing the closures would force patrons to walk an extra block down Market Street or cut through UN Plaza, an area plagued by grime and open-air drug trafficking.

“The concern was not knowing how far people would have to go,” said Melanie Smith, president of San Francisco Performances.

She acknowledged that Civic Center’s environment has significantly improved since last year, when the late Mayor Ed Lee partnered with BART to clean the surrounding streets, beef up police patrols, install used needle kiosks and direct homeless people to services. These days it’s abuzz with children climbing the jungle gym in Hellen Diller Playground, people dancing zumba beneath the sycamore trees or sipping espresso at the new Bi-Rite Cafe.

Shutting off another valve to the subterranean station may help keep drug use and squalor at bay, though that wasn’t the impetus.

Once the stairwell and concourse area are barricaded, BART will install a 34.5-kV power cable — a dangerous task that requires work crews to be separated from riders.

It’s the first step to build an electrical facility that will supply more power to the system’s third rail. Funded by BART’s $3.5 billion Measure RR general obligation bond, the substation comes alongside other upgrades, including a new train control system and a fresh fleet of Bombardier rail cars.

In the coming years, BART will boost the length and frequency of its trains, from 24 per hour in either direction of the Transbay Tube, to 30 per hour. The rail line expects to boost capacity in the tube by 40 percent during peak commute hours, said BART spokesman Jim Allison.

“Though the station will lose two entrances, there are others still available that are located much closer to the platform, fare gates and ticket vending machines,” Allison said. “Customers will gain the benefits of more frequent and more reliable service when the new substation is completed.”

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan