In addition to a wide swath of proposed cuts in federal grant and loan programs, the Reagan administration is considering sharp cutbacks in two of the most widely used government subsidized programs -- the Postal Service and Amtrak.

A budget document prepared by Office of Management and Budget Director David A. Stockman proposes new standard of self-sufficiency for Amtrak's trains that would amount to a wholesale reduction of routes and price hikes for tickets that could as much as quadruple the old rates.

Another part of the document makes it clear that Saturday mail service has no place in hard times. "The possible reduction of service to five-day delivery is a symbol of the seriousness of the fiscal austerity being imposed by reductions throughout the federal government," according to a section dealing with Postal Service subsidies.

The proposed Amtrak changes amount to "a fundamental philosophical change," according to one Amtrak official. The budget document itself notes: "Passenger trains, while part of the country's heritage and history, have little place in a federal budget which is heavily in deficit."

President Reagan's budget architects hope their proposals will make good on his campaign promise to cut federl spending and get the government books to balance, but in putting the proposals together they indicated not only little regard for history, but also little respect for popularity.

The $26.2 billion in contemplated budget cuts contained in the Stockman document, obtained over the weekend by The Washington Post, cuts across the board -- from food stamps and Medicaid to endowments for the arts and credit subsidies under the Export-Import Bank. As much as $20 billion more in proposed reductions are expected to take shape before Reagan sends his revised budget to Congress.

What the document proposes for Amtrak is drastically stepping up the share of the costs that individual train routes are supposed to cover from revenues and sharply reducing federal subsidies for the trains.

"In 1981, passenger fares will cover only 40 percent of Amtrak's total operating costs. Federal subsidies represent 60 percent of the operating costs and 100 percent of the capital costs," the proposal said.

As things now stand under the Amtrak Act of 1979, Amtrak is supposed to cover 50 percent of its operating costs from passenger revenues by 1985.

Reagan's budget analysts recommend moving that standard up to the end of 1982 and applying in on a route-by-route basis instead of systemwide. By 1985, each train route would be required to cover 80 percent of its costs from revenue.

"In the best, most efficient service in the world you might find one or two trains that meet the goal," said Lawrence A. Gilson, Amtrak vice president for government affairs. No other national system in industrialized countries such as Japan and France are that efficient, he said. "The decision has to be made whether the government is going to continue to support passenger rail service as every other industrialized nation does."

Even if passengers were willing to pay up to twice as much as they now pay for train tickets, approximately a third of the long-haul routes and about half of the short-haul routes would be dropped in 1982. More trains would be dropped as Amtrak chugs along toward the 80 percent standard in 1985.

The documents estimated some of the price increases needed for certain routes to meet the 50 percent requirement in 1982. They range from an estimated increase of 26 percent for the heavily used New York-Florida route to a 394 percent increase needed for the Washington-Cincinnati route.

"Average systemwide operating losses are now at about 16 cents per passenger mile. On many routes the average ticket subsidy is $60-$70 per person and it reaches $137 per ticket (the Southwest Limited)," the budget document said in arguments it marshaled for budget cuts for the rail service.

"These subsidies are a grossly inefficient use of tax dollars, especially when train travel represents only about 1 percent of intercity travel," the report said.

The budget proposal also argues that Amtrak "is not particularly energy-efficient because of low passenger load factors, especially outside of the Northeast Corridor. Passenger trains are likely to become comparatively less efficient as automobiles and airplanes make major improvements in fuel economy." Amtrak officials dispute that.

The budget cuts proposed by the Reagan team begin with a $25 million trim for this fiscal year and increase to $900 million in fiscal 1985.

Gilson said he has not yet seen the document but added, "The numbers I have heard would give us acute heartburn."

On the question of Postal Service subsidies, the Reagan administration plan calls for phasing out the public service subsidy at a slightly slower rate than that proposed by the Carter administration. But it also calls for a substantial reduction -- from $789 million in 1981 to $500 million in 1982 -- in outlays to absorb the Postal Service's losses because of lower mailing rates charged to nonprofit organizations.

"The public may complain about any simultaneous increase in rates and a reduction in service," the document said, nothing that a decision on a 5-cent rate increase for first class mail is pending. It also noted, in a discussion of probably reaction, that "the powerful postal unions oppose five-day delivery and will resist reductions in federal support as new wage contract negotiations are being started."

The postal unions begin negotiations on a new contract in March, a congressional aide said.