Whether fearful of gasoline shortages or grounded by the six-week strike against United Airlines, Americans are rediscovering the passenger train.

Advanced train reservation for the next two weeks have jumped 40 percent from the same period last year. On the route between San Diego and Los Angeles last weekend, half of the 22 scheduled trains had standing customers-648 standees in all.

Although the Southern California train boom obviously reflects the severe gasoline pinch in that state. Amtrak is experiencing similar business increases elsewhere.

On the East Coast, busy Boston-Washington corridor service is not so affected by the reservations crunch because there are frequent trains and many riders pick up tickets just before departure. But New York-Florida trains are booked to near capacity and the Crescent between New York-Washington and New Orleans is booked to 59 percent of capacity for the next two weeks and 48 percent for the next six weeks.

The Southwest Limited, between Chicago and Los Angeles, was sold out on eight occasions during May 1978. Of 62 trains during this month-one in each direction daily - 18 already have been booked full and "several more are on the fringe" of being sold out, Amtrak spokesman Joseph Vranich said yesterday.

Nationwide, the volume of telephone calls to Amtrak reservation numbers on Thursday was up 50 percent from last year to 219,000. So swamped was the railroad's reservation-computer center near Philadelphia, that American Telephone & Telegraph Co. warned Amtrak that it was overloading that metrpolitan area's long-distance telephone capacity and burdening other customers.

For the first ten days of May, Amtrak sold 316,700 tickets-up more than 14 percent from the 1978 period. But, since more than one person often travel on one ticket, actual ridership figures are expected to be up even more sharply.

To the people at Amtrak, it all adds up to an environment very similar to the Arab oil embargo in 1973 and 1974. Then, Amtrak's cars were often crowded to capacity by persons who never had ridden a train. To supplement the aging fleet that Amtrak had inherited from private railroads when it began service in 1971, the national rail firm borrowed even older and unused commuter cars from the New York metropolitan area.

Except for the over-busy telephone lines, however, Amtrak to date has been able to handle most of the increased ridership interest in 1979. But Vranich warned that the commuter cars since have been scrapped and Amtrak has no backup fleet to use if business continues to swell.

"This is beginning to look like the oil embargo all over . . . we're going to be accused of more incompetency again," as customers find crowded trains or delayed service, Vranich said. He also noted that for three consecutive years, Amtrak has requested funds for 350 new passenger cars and that, each time, such appropriations have been rejected.

Ironically, the reservations are pouring into Amtrak as the government-subsidized company-formally called National Railroad Passenger Corp.-is facing a massive cutback in route miles by next fall.

Unless one house of Congress acts within two weeks to veto a Carter administration plan made public in January, Amtrak's 27,000 miles of routes will be slashed by some 40 percent on Oct. 1. Although Senate and House committees have acted to amend the administration plan slightly by keeping a handful of routes scheduled for abandonment, there was no prospect yesterday of any signficant Capitol Hill movement to veto the basic restructuring.

Meanwhile, to cope with the current reservations boom, Amtrak has hired 60 new reservations agents. They are in training and should be manning telephones in three weeks.