The recovering economy, high gas prices and growing environmental consciousness are driving record ridership on BART. But the surge in riders - about three times the increase that was expected - could also bring problems if the transit district doesn't act to increase its capacity and rejuvenate its aging infrastructure.

"We could be looking at half a million riders a day in five years and three-quarters of a million a day a decade after," said Paul Oversier, BART's assistant general manager of operations. "Do we want to position ourselves to accommodate those riders, or do we want to put ourselves in the position of driving them away because we can't provide the service we do today?"

BART gives an average of 390,000 rides each weekday, an increase of about 6 percent over the prior year, agency officials said. BART's budget had anticipated a rise of 1.8 percent.

With the transit system beginning its fifth decade of service, dealing with its growing ridership while modernizing the aging system was the chief topic at a two-day workshop at a San Francisco hotel.

BART officials told the agency's Board of Directors that to handle growth without diminishing service it needs to:

-- Bump up its existing orders of new train cars by 225, boosting the size of its fleet from the current 669 to 1,000.

-- Increase the capacity of the platforms in the station's busiest stations, Embarcadero and Montgomery.

BART's growing ridership already means that many passengers must stand, often for up to 30 minutes. On weekday trains heading into San Francisco, trains usually fill by North Berkeley, Lafayette, Castro Valley and Hayward on the transit system's various lines.

"We are experiencing standees further and further down the line," said Bob Mitroff, BART's manager of fleet and capital planning, "and it's only going to get worse."

The most crowded trains during typical commutes carry an average of 107 people - with 47 standing and a maximum of 122. Each standing rider typically has about 4.9 square feet of space. But by 2020, if BART grows at a 3 percent rate, a car will haul an average of 130 people with a maximum of 149, leaving them just 3.4 square feet to stretch out.

"At 130," Mitroff said, "you're going to get into Yogi Berra territory where nobody's going to ride BART because it's too crowded."

To avoid that, BART will need to build its fleet to 1,000 new rail cars, which will cost the agency $837 million - if state and federal sources pick up 75 percent of the cost as they are doing with the agency's current order. BART has ordered 410 cars, and has an option to buy another 365 cars. New cars are expected to start arriving in 2017. If BART were to order 1,000 cars, a decision it may need to make within five years, they could all be delivered by 2023.

But BART is also experiencing crowding in some of its stations, particularly Embarcadero, which, with Montgomery, is the busiest of its 44 stops. The crush of weekday commuters is heavy on a daily basis but about once every three or four months is so severe it forces the temporary closure of the platform and, sometimes, the station. The platform at Embarcadero is 7 feet narrower than the one at Montgomery, Mitroff said.

Kim Armanasco, an insurance broker who uses the station daily, said something needs to be done.

"In the last few weeks, sometimes it's been so crowded I've been worried that someone was going to get pushed onto the tracks," she said. "Sometimes there's a lot of pushing and shoving."

BART officials are considering everything from building new "saddlebag" platforms that would sit on the opposite side of the tracks to installing platform screen doors like those on airport people movers to removing or replacing benches to installing high-capacity elevators or running skip-stop service, in which commute-direction trains would stop at only one of the stations.

Adding the "saddlebag platforms," which Mitroff said are likely to be needed eventually, would cost $615 million, based on a 2009 estimate. One would be added at Montgomery with two at Embarcadero.

BART officials also discussed the need to modernize its automatic train control system, which could increase the number of trains it can send though the Transbay Tube from 24 to 30 an hour. And the rail system also needs to expand and modernize its train maintenance yard in Hayward and start investing more in maintaining and improving its infrastructure. That need is estimated at $6 billion over the next 10 years and $17 billion over the next 30.

The improvements would cost billions. State and federal funds will cover some, but not all, of those needs, which means directors will need to decide how to raise that money. Fare increases and tax measures are likely options.

Commuters at Embarcadero station Friday evening agreed with the need for improvements but weren't sure they could support higher fares or taxes.

Matt Rodriguez, a banker who commutes from Castro Valley, said BART fares are already too high but that he might consider a moderate tax increase so that the burden of the improvements would be shared.

"It shouldn't hit just me as a rider," he said.

Armanasco was more reluctant, saying that taxes are already rising.

"I understand the need," she said. "But I'd like them to figure out another way."