Metro staff planners have recommended that Yellow Line subway service between Gallery Place and National Airport begin next April, about six months earlier than previously proposed, using cars diverted from other lines.

The proposal, which must be approved by the Metro board before it can take effect, is a shift in thinking for the staff, which had argued that Metro's 300-car fleet should not be stretched to open the new track.

The staff had earlier said Yellow Line service should not begin until the arrival in late 1983 of the first of 300 new rail cars Metro has ordered.

Metro operations chief Theodore Weigle said Metrorail's lower than expected ridership convinced analysts that shorter trains could serve existing stations adequately. The staff now estimates that more than 20 cars could be freed for the new track and 18 for special reserve trains to improve reliability.

Under the new proposal, Red, Blue and Orange line trains would be trimmed beginning in January, creating more crowding for passengers on those lines. Eight-car trains would disappear from the Red Line, and the Blue and Orange lines, now using a mix of four- and six-car trains, would have more four-car trains.

If these trains could handle passenger loads at stations now open and if reliability did not suffer, the proposal said, Yellow Line service across the Potomac River would start in April, using Metro's bridge at 14th Street.

Under the new staff plan, new rail cars would be used to extend Yellow Line service south from National Airport to Huntington late in the year, bringing the first trains to Alexandria.

Metro board members cautioned yesterday that the plan is only a proposal and could be turned down. But Virginia board members, who have seen the opening dates for their stations pushed back repeatedly, are applying heavy political pressure for quick action.

Board members representing Maryland, whose constituents would not benefit from the new service, have expressed concern that moving fast may worsen Metrorail's reliability problems. Metro officials believe that frequent breakdowns are partly to blame for lagging ridership.

Weigle said the new plan would improve reliability by creating three "gap trains" to be stationed around the system to go into service if other trains break down.

In addition, Metro planners believe that the roundabout route Blue Line riders bound for Virginia now take, going upstream and then back downstream, uses cars inefficiently. Many riders, it is assumed, would change at L'Enfant Plaza to Yellow Line trains for the shorter trip across the river, meaning fewer cars would be needed on the Blue Line.