BART General Manager Grace Crunican has all but slammed the door on the idea of building a station near Oakland’s Howard Terminal — the waterfront location that Mayor Libby Schaaf has been promoting for a new A’s ballpark.

It’s not a “viable alternative,” Crunican told the team, for two main reasons.

For starters, it would be located between the West Oakland Station — gateway to the Transbay Tube — and the Oakland Wye underground connector downtown that’s used by four separate lines. It’s the most heavily trafficked section of BART in the East Bay.

Putting a station at the site northwest of Jack London Square, Crunican told A’s President Dave Kaval in a letter last week, would “permanently reduce speeds and introduce uncertainty ... where we can least afford it.”

To say nothing of the problem of building a station in the middle of a vital BART artery while still running trains there. Crunican said service would have to be shut down for a time, a prospect that she said was not realistic.

“By the time we would be able to design and build such a project, I’m sure the A’s would have had at least 10 winning seasons and a World Series win in the new stadium,” she wrote.

She didn’t even mention the cost — or who would pay it.

BART’s assessment hardly comes as a shock to the A’s. For years, the team has maintained that the Howard Terminal location is unworkable for a variety of reasons, including the lack of easy access to BART.

Still, Crunican’s “forget about it” letter is the second blow in three months to the A’s efforts to find a new home in Oakland.

In December, Peralta Community College District trustees shot down the team’s proposal for building a ballpark on school property next to Laney College, after faculty and students objected.

That site was within walking distance of Lake Merritt BART Station, which was one of the reasons the A’s liked it. When the Peralta trustees turned their thumbs down, the team agreed to take another look at Schaaf’s favorite spot — including asking BART to weigh in on the possibility of building a Howard Terminal station.

The A’s had no immediate response to BART’s stance. But we’re told the team hasn’t ruled out other transportation options, such as increased bus service or even a light rail line, to make the site work.

In fact, Crunican wrote that BART would be happy to work with the A’s in figuring out how to “connect fans to the 12th St/Oakland City Center and West Oakland BART stations” if the team pursues Howard Terminal.

As for Schaaf: “It’s a disappointment, but it needn’t derail the project,” she said of Crunican’s letter. “I know the A’s are working hard and pursuing all options to stay and build a site in Oakland.”

Meaning, as she sees it, the A’s still might want to consider rebuilding at the Coliseum — a location the team has said doesn’t pencil out.

Fired up: The San Francisco firefighters union has opted not to do a ranked-choice endorsement and instead is going “all in” for Supervisor London Breed in the June mayoral race.

The decision isn’t going over well with supporters of former Supervisor Angela Alioto, who finished second in a vote of the rank and file. But union President Tom O’Connor noted that it was the members themselves who made the call in a show-of-hands vote.

The endorsement followed a candidates forum attended by about 300 of the union’s 1,500 members, where Supervisor Jane Kim, former State Sen. Mark Leno, Alioto and Breed were questioned on everything from homelessness and drug overdoses to how the reshaping of city streets to accommodate bikes and pedestrians could affect fire trucks and ambulances.

O’Connor said union rules precluded him from giving the tally on the vote count, but we know Breed came in first, followed by Alioto and Leno.

All the candidates have histories of supporting the firefighters, so many thought the union would endorse at least two of the four. Under the ranked-choice system, voters list three candidates in order of preference, with the bottom finishers’ votes being redistributed until someone gets a majority.

“I was surprised, but they decided to go all in with Breed,” O’Connor said.

The union’s endorsement is a holy grail for local office seekers. Firefighters are especially popular on the city’s west side and, unlike members of the police union, they actually walk precincts in support of their candidate.

Plus, the firefighters have a political action committee, which lets them collect big money from outside contributors and spend it in support of their pick.