Five railroad tank cars were pulled and pushed onto a siding in Northern Virginia yesterday, carrying liquid propane from the Midwest for Washington Gas Light Co.

The rail transportation program is one element in a diverse effort designed to supplement natural gas from interstate pipelines, which account for the bulk of area supplies.

Washington Gas, which is able to draw on the supplemental resources during periods of peak demand, stores propane on its own underground in Kansas and Mississippi.

Recently, the local firm leased 112 railroad cars to carry the propane to storage facilities here, after which it can be converted for use in a distribution system that serves metropolitan Washington and other areas of Virginia, Maryland and West Virginia.

At Ravensworth, a Fairfax County facility served by Southern Railway, Washington Gas can store up to 13 million gallons of propane in a manmade, underground cavern. At a separate facility in Rockville, the firm also has a rail siding and can store up to 3 million gallons of propane in bottles underground.

The utility also owns an underground storage facility in West Virginia, capable of delivering 50 million cubic feet a day of gas to local firm's distribution network; with depleted reserves, the West Virginia field currently is providing 30 million cubic feet daily. Other natural gas is stored in Rockville.

Among other resources, WGL has been buying synthetic gas, which is shipped here by pipeline from a plant in Ohio. The firm's own program of drilling for new gas has produced 1.5 million cubic feet a day - which began flowing here this week from a successful well in Louisiana. When demand is greatest, the company manufactures gas from oil at a plant on the Anacostia River.

Overall, WGL has been selling about 100 billion cubic feet of gas a year recently. Next year, a major boost in available supplies is expected when WGL shares in imports of liquefied natural gas from Algeria, to be landed at a new terminal in Cove Point, Md. The local firm expects to receive about 8 billion cubic feet a year of the imported gas.

All of the supplemental gas resources being added by Washington Gas cost more than the current price of natural gas in the interstate markets. For example, the propane costs about $4.50 a thousand cubic feet compared with the interstate wellhead price of $1.44.