According to conventional wisdom, Americans have given up on passenger trains to such an extent that the rails never again will rival highways for intercity travel.

But his month-not counting Memorial Day-Amtrak carried more passangers between Los Angeles and San Diego than in previous rail history.

Amtrak's San Diegans carried 128,037 riders from May 1 to May 27, breaking an all-time record of 125,000 passengers carried during the month by the Atchison, Topeka & Santa Fe Railroad during World War II, when rail travel was at a peak.

Moreover, on the 8:30 a.m. departure for San Diego on May 27, all seats were occupied and there were 280 standing passengers, even though Amtrak had attached 12 passenger cars instead of the normal 5.

These new figures, distributed to Amtrak's employes on Tuesday night, point to an unusual situation that has developed in California because of gasoline shortage.

But the new statistics also show a nationwide pattern of sharp increases in rail passenger business in the wake of congressional decisions to permit the Carter administration to begin planning for a 43 percent cutback in Amtrak routes on Oct. 1.

According to the ridership figures:

The Memorial Day weekend was one of the biggest on record for Amtrak, although exact data has not been compiled. Standees were reported on 39 trains, and such key stations as the rail hub at Chicago and Union Station here reported heavy loadings on virtually all trains.

Trains with standing space only included Boston-Washington routes and the Hilltopper south of Washington, because of heavy travel to the Kings Dominion amusement park near Richmond.

Last Friday, the Amtrak reservation system contained the names of 200,000 persons seeking future space - 90 percent higher than advanced reservations at this time last year. Dollar volume of sales at the five main reservations centers now exceeds $1 million a day compared with $600,000 a year ago, and ticket sales through automatic machines are up 18 1/2 percent.

Amtrak had expected the reservations overload to ease a bit after United Air Lines resumed some of its services Monday but spokesman Joseph Vranich said yesterday that the reverse has happened: Phone volume still is on the increase. Of callers to Amtrak reservation centers last week, 1.4 million got busy signals - a situation expected to be helped this week as 100 new reservations clerks started their jobs after several weeks in training.

Meanwhile, there is no clear indication about the future of Amtrak. Member of the House and Senate are preparing various amendments to Amtrak authorizations bills - all of which plan to keep some trains in business that now are scheduled to be halted after Sept. 30. Final decisions may not be made until late in the summer because pro-Amtrak legislators are counting on a strategy of allowing the enery crisis to demonstrate Amtrak's advantages, especially if gasoline remains in short supply during coming weeks.

According to the Carter administration plan, two dozen trains are due to be stopped and Amtrak plans to post notices on such routes 30 days before discontinuance.

Typical of the support for specific trains now building on Capitol Hill, an aide to Rep. Richardson Preyer (D-N.C.) said this week that Amtrak's Crescent between Union Station and New Orleans may still have a chance for survival if ridership gains continue. The Crescent is among trains due to be stopped under the administration plan.