Two of BART's busiest San Francisco stations could be on track for a $900 million expansion, complete with new tunnels, elevators and extra platforms to boost rider capacity.

The rebuilding of the Embarcadero and Montgomery Street stations would require tearing out the existing walls, installing new platforms, boring additional tunnels for staircases, and putting in extra elevators.

For added safety, the new platforms would have automated sliding glass doors that would open when the trains arrive.

The whole job could take more than five years, but the stations would remain open during that time.

The plan, floated by staffers at a BART directors retreat last month, is intended to accommodate the transit system's ballooning ridership. BART carries an average of 393,000 riders per weekday, up 6 percent in the past year.

Of those, more than a third of the system's total riders - about 150,000 a day - get on and off at the Embarcadero and Montgomery stations.

When the San Francisco stations were designed in the late 1960s, "I don't think anyone predicted that ridership would be at the levels they are today," said BART spokeswoman Alicia Trost.

In five years, BART predicts, weekday ridership could hit 500,000 - making the two stations' improvements "mission critical," according to a staff analysis.

BART General Manager Grace Crunican has already proposed buying as many as 1,000 additional cars and a new control system to increase the number of trains that can run through the Transbay Tube to 30 an hour in each direction, from the current 24.

But to speed up the trains, BART needs to relieve the bottleneck in the Financial District stations. Any big glitch in the system, and the platform gets so crowded now at rush hour that service can actually slow down, as operators wait for the crush of people to sort themselves out.

Hence, the idea of new "saddleback" platforms - one in each direction at Embarcadero and one on the eastbound side at Montgomery - to allow riders to board and exit simultaneously from both sides of a train.

A BART consultant's study has concluded the proposed platforms "could be built without significant disruption to BART service" - though much of the work would have to be done at night, and there might be times when trains in both directions would have to be run on a single track.

Still, there are skeptics.

"I presume what they are doing is needed to relieve pressure - because there is pressure," said Kofi Bonner, a former Oakland city manager and now a prominent developer, whom we found waiting for a train at Embarcadero. "But the price seems a little high."

For the record, BART has officially been using a $615 million price tag from planning estimates four years ago, but says the cost would grow to about $900 million by the time work begins in seven years or so.

The immediate challenge is how to scoop up federal money.

Metropolitan Transportation Commission spokesman Randy Rentschler is hopeful, saying the added station capacity promises BART and the public "some of the best bang for the buck."

Heart and arrow: What better way to celebrate Valentine's Day than a giant civic rally on the steps of San Francisco City Hall to condemn global violence against women and girls?

Mayor Ed Lee, District Attorney George Gascón and Board of Supervisors President David Chiu will all be on hand, and the invites have gone out to the rest of the supervisors as well.

As for Sheriff Ross Mirkarimi, who's still on probation for a domestic violence involving his wife?

"We haven't had a chance" to invite him, said Aimee Allison, spokeswoman for the Department on the Status of Women, which is helping organize the gathering.

She quickly added: "Every elected official is welcome to be there - it's a public event."