The Washington area's troubled Metrorail system picked up some surprising congressional support yesterday at about the same time it suffered a double setback in the suburbs.

Rep. William H. Natcher (D-Ky.), long regarded as Metro's most effective critic on Capitol Hill, endorsed a proposed federal subsidy to pay interest on nearly $1 billion in bonds that Metro sold to raise construction funds.

Natcher spoke out at a hearing of the House District Appropriations Subcommittee, of which he is chairman. Unlike past years, when the atmosphere was one of the suspicion sometimes verging on hostility, yesterday's session was friendly.

The big difference was the first appearance of Theodore C. Lutz as Metro general manager, replacing Jack-Graham, a retired Army general, who served in that post from 1967 to 1976. Natcher frequently mixed praise for Graham personally with criticism of testimony he pronounced to be unreliable.

"I like the way you answer questions," Natcher told Lutz, adding later: "We've had a difficult time (in the past) getting answers to our questions."

During the hearing, Rep. Clair W. Burgener (R-Calif.) voiced personal enthusiasm for Metro and urged its officials to enlarge the number of automobile parking spaces at outlying stations above the 29,000 now planned. It might be wise, Burgener suggested, "to spend $100 million (more) to park the cars."

Lutz said he already has instructed Metro staff planners to explore an increase in parking facilities.

Natcher's support for a federal subsidy of Metro's bond interest came the morning after the Alexandria City Council and the Prince George's County Council, backed off from the support sought by Metro for a bond interest payment due July 1.

Alexandria refused to approve $173,785, and Prince George's $528,949, to help pay $12 million in interest that Metro is supposed to make July 1 to the buyers of the first batch of revenue bonds it sold in 1972.

When sold, it was expected the bonds and their interest would be paid off from fare collections. These, however, are not expected now to cover operating costs.

Since the bonds are federally guaranteed, the Department of Transportation must - by law - make up for Metro's almost certain default by drawing the money from the U.S. Treasury and paying the bond holders itself.

Metro officials asked both the Ford and Carter administrations to grant an 80 per cent subsidy of the interest payments, but have received no answer.

The funds that Alexandria and Prince George's refused to grant would have gone toward the 20 per cent local share, $3.3 million, under that proposed formula.

Natcher's subcommittee would not have jurisdiction over the enactment of any law that may be needed to adopt an 80 per cent federal subsidy. But his support, in view of his past strong - and usually effective - criticism, could be influential among House members who usually pay little attention to District of Columbia affairs.

Both the Alexandria and Prince George's actions were based on local legal rulings that they had binding obligations to make the payments.

Members of both Councils have been among the region's most vocal critics of Metro's rising costs, with some of them openly seeking to extricate their jurisdictions from further involvement in Metro Construction.

Their criticism is reminiscent of critical statements made by Natcher over the years, starting in 1967, two years before the Metro groundbreaking.

Natcher accused Graham over the years of consistently understating cost estimates. At one point, when Graham was insisting a 97-mile system could be built for $2.5 billion, Natcher supplied his own prediction of $4 billion to $5 billion.

"All we said (to Graham)," Natcher recalled yesterday, "was, we just want you to tell the people the truth."

Now, with Metro's official forecast for an expanded 100-mile system topping $5 billion, Natcher told Lutz he thinks a completed Metro will cost between $6 billion and $7 billion and will "take a tremendous subsidy."

Lutz did not directly challenge the prediction, but he reminded Natcher that Metro is undergoing a restudy that could cut the system's size and its cost.

He agreed that a substantial operating subsidy would be needed, with the dollar amount depending upon the level of fares and the amount of service provided. This, he said, is "a social policy decision."

On the repayment of the revenue bonds, Lutz agreed with Natcher that "a prudent man would have to conclude you could not retire these (now) from the fare box."

It was at this point that Natcher said he would support a federal subsidy of the bond interest.

Lutz and other Metro officials appeared at the hearing to testify on the District of Columbia government's request for $8.3 million in Metro construction funds, $25.1 million in Metrobus subsidies and $2.8 million in future bond interest payments during the fiscal year that will start Oct. 1.

Responding to questions, Lutz said it now seems a good bet that 60 miles of the rail system actually will be constructed under a stopgap financing program recently agreed upon by the area's political jurisdictions. Most of the money is coming from funds transferred from abandoned interstate highway projects.

Although Metro will open two more lines after this year, and will have 23 miles in operating. Lutz said it will be 1978 before consideration is given to extending the hours of train service - now from 6 a.m. to 8 p.m. on weekdays only.

Lutz disclosed that patronage on the existing five-mile line is edging up to 35,000 fares a day with the arrival of good weather and the recent opening of the second entrance to the Dupont Circle station. Before the line opened 13 months ago, only 10,000 daily riders were expected.