Top officials from the Los Angeles, Philadelphia and Miami transit systems told Congress yesterday that they are trapped between the federal government and an unstable bus manufacturing industry in their efforts to buy more buses.

Jack R. Gilstrap, general manager of the Southern California Rapid Transit District, said that ridership on his Los Angeles system had increased 25 percent in the last two weeks as long lines formed at southern California gasoline stations.

New demand such as this, being reported by many transit systems across the nation, comes at a time when bus purchases have been decreasing because of rising costs and the ill-fated federal requirement that all new buses be the so-called Transbus, a low-floored vehicle that was intended to provide easier access for elderly and handicapped riders.

Los Angeles, Philadelphia and Miami had joined to place the first order for 530 Transbuses. On May 2, when bids were to be opened, there were no bidders.

Instead of rebidding the Transbus Contract, spokesmen for the three cities said yesterday, the Transbus concept should be restudied. At the moment, purchases of regular buses are being permitted.

"Nationally, the transit industry in the United States requires about 5,000 buses per year to meet current demand," Gilstrap said. "Due to a multitude of problems . . . less than 2,500 buses were built and delivered in 1977 and I am told there were fewer than 2,000 last year."

Furthermore, he said, new buses were designed before energy problems arose. The new buses are less efficient, carry no more people than the old ones and cost more to buy, he said.

Peter Young, of the Philadelphia area's Southeast, Pennsylvania Transit Authority, questioned the effectiveness of proposed federal regulation requiring that all buses bought with federal aid be equipped with mechanical lifts for wheelchairs.

He pointed out that the lifts are unreliable as Metro has discovered on a few test buses in Washington add about $15,000 to the price of each bus, and that "even if all regular route mass transit vehicles were fully accessible, there would still be a need for special services . . . to provide mobility to the handicapped."

Since the Transbus contract fell through, Los Angeles has decided to buy 900 other buses. The requirements for lifts will add $1.3 million to the cost of these buses at a time when the Carter administration is holding down appropriations for federal aid to transit. The federal government provides 80 percent of the cost of a new bus.

The would-be Transbus buyers made these comments yesterday at a hearing of California Democrat Norman Y. Mineta's Oversight Subcommittee of the House Public Works and Transportation Committee.

The committee is looking at problems transit authorities encounter in buying buses and subway cars. Only two U.S. manufacturers - General Motors and Grumman Fibixle - now build buses and only one, The Budd Co., however, is now German-owned.

Representatives from both GM and Grumman Fixible testified that they had declined to bid on Transbus because the federal written specifications presented too great a technical risk.

They also insisted that the buses they sell today - known in the industry as Advanced Design Buses - meet most aims of the Transbus program except for a low floor. The big advantage of the low floor is in reducing the steps a person must climb to enter a bus.