An unrelenting sun continued to broil the Washington area and much of the rest of the nation yesterday with no immediate relief in sight for the heat, the humidity or the pollution.

Forest fires raged yesterday in Maine, Utah and Montana. Railroad tracks buckled in 100-degree heat near Paintsville, Ky., causing a freight train to derail. Cucumber, cantaloupe and tomato crops in rural Virginia and Maryland were reported in danger as drought conditions worsened.

Officials in New York and other eastern cities urged residents to conserve dwindling reserves of both electricity and water.

Here, the Metropolitan Washington Council of Government extended its pollution alert for the fifth straight day, and the area's power companies reported record consumption of electricity to meet the growing demand for air conditioning.

The temperature yesterday reached an official high of 96 degrees at National Airport in midafternoon, 6 degrees short of the record 102 degrees set for the date in 1930. This was little solace for area residents who have been enduring humid, dirty,, 90-degree air for almost three consecutive weeks now.

Dulles Airport, usually cooler than Washington, recorded 98 degrees yesterday. Baltimore and Martinsburg, W. Va., both registered 101 degrees, and New York reported a record-breaking 102 degrees.

Portions of the roadway at North Capitol Street and Missouri Avenue buckled from the heat during the afternoon rush hour, requiring police to direct traffic around impassable sections.

The Potomac Electric Power Co., which services 460,000 customers in Washington parts of Montgomery and Prince George's counties in Maryland and a small section of Arlington, reported a record peak load at 5 p.m. yesterday of approximately 3.835 megawatts, more then 100 megawatts over the previous record set July 6. Likewise, the Virginia Electric & Power Co., said it set a record Monday of 7,709 megawatts for its 1 million customers in Virginia and parts of North Carolina and West Virginia. Consumption figures for yesterday were not immediately available.

For the first time this summer, Pepco also went on a "voluntary cutailment alert" Monday when it appeared that consumption might begin to eat into the reserve capacity set aside by the Pennsylvania-New Jersey-Maryland (PJM) grid, a regional system in which 12 power companies including Pepco pool their energy.

Curtailment became unnecessary, however, said Pepco spokesman John Grasser, when consumption - which is usually highest in midafternoon - did not reach expectations.

Monday's peak load was 3,654 megawatts between 3 and 4 p.m., well below the July 6 record. Pepco officials were concerned nevertheless because units at two of their six generating plants were shut down for repairs as were 11 other units in the PJM system as a whole. There are 117 power plants with 547 units in the entire PJM system.

One of the two Pepco units, a generator at the Chalk Point plant in southern Prince George's County, was returned to service late Monday, Grasser said. The other, a unit at Morgantown. Md., has been out of commission since June 25 with "turbine trouble." Grasser said, and will not be back in operation until Aug. 5.

The "alert" action Monday fell short of more serious steps taken by Pepco in the past when the PJM reserve capacity was endangered.Last Jan. 17, during one of the coldest periods in the city's history, voltage was reduced 5 per cent to conserve energy. The last time voltage was reduced in the summer was July 9, 1974, when a 5 per cent reduction was imposed.

COG extended its pollution alert to 3 p.m. today after registering a "very unhealthy" 135 air quality index reading in Bethesda at 3 p.m. yesterday. Five of COG's seven air quality monitoring stations recorded 100 or higher on the index yesterday, according to COG spokesman William H. Gilbert.

Gilbert reiterated COG's warning to persons with allergies, respiratory ailments and heart conditions to stay inside and urged commuters to use car pools or public transportation to reduce air pollution by cars.

Forecasters at the National Weather Service called for more hot, muggy weather today and Thursday with high temperatures of 95 to 100 on both days.

The weather service blamed the current heat wave on an enormous but stagnant high pressure system extending from the Mediterranean Sea westward across the Atlantic and the eastern United States to the Rocky Mountains. Relief is not expected here before Friday - if then - weathermen said.