As railroad cabooses begin rolling off the track and into museums across this country, a young Virginia company has found a new overseas market for the little cars.

C&S Transit Corp., based in Chesapeake, is putting the finishing touches on the last of eight specially equipped cabooses it is building for the Saudi Arabian Railways Organization. Gleaming green and white and smelling of fresh paint, with the crossed-swords-and-palm-tree symbol of Saudi Arabia emblazoned on the sides, the cars may be the last cabooses ever built in the United States.

U.S. railroads are phasing out cabooses, replacing them with shoe-box-size microelectronic devices that can monitor a train's movements and air brakes. In this country, the new instrument has rendered unnecessary the small cars from which crew members watched trains by eye for more than a century.

But travel across the vast Saudi Arabian desert presents challenges similar to those met by the caboose when railroads first conquered the American frontier. While U.S. trains today travel through populated areas and can stop frequently for repairs, Saudi Arabian trains may go for 18 hours at a stretch without passing a station with adequate maintenance facilities, said Bruno Puntel, a C&S Transit project manager. The new cabooses will serve as mobile maintenance shops, carrying crew members for possible repairs.

The cabooses are the first cars C&S Transit has built from scratch, marking a new direction for a company that built its business on refurbishing mass-transit and railroad cars.

C&S was founded four years ago out of the rubble of Amtrak's Auto-Train Corp., which operated a maintenance service subsidiary in nearby Portsmouth. When Auto-Train went bankrupt in May 1981, a group of investors bought some of the Portsmouth plant equipment, hired some Auto-Train employes and set up a company to build and service rail cars out of a cavernous warehouse on the Elizabeth River.

Once a yacht-building yard, the warehouse still echoes with the buzz of power drills and glows with the light of welding torches, but now it is filled with old locomotives, passenger rail cars, box cars, and cabooses in various stages of construction.

C&S Transit's eight cabooses were designed and constructed according to the very precise specifications of the Saudi clients, said Lawrence H. Fort, president of the company.

Built to withstand desert heat of 140 degrees Fahrenheit, the cabooses are painted white on the roof and underneath the car to reflect the sun's glare from the sky and sand. Window glass one-quarter of an inch thick is specially tinted to keep out the sun, and a powerful state-of-the-art air conditioner refrigerates the interior.

Six inches of padding between the steel exterior and the plywood interior walls provides further insulation to keep the heat out. "There's not one inch that's not insulated," said Tom Santomieri, a C&S project manager.

Each of the 30-foot-long cabooses also includes a small toilet for the long haul, two cushioned seats and a cushioned couch, and a luggage compartment with wide, spacious shelves. "I can't imagine that they won't be used as bunks," Fort said of the shelves.

Also according to Saudi specifications, the rubber-tile floor is painted bright apple green, a couple of shades darker than the lime green walls. The last two cabooses, to be shipped in June, will include wall-to-wall carpeting and small kitchenettes, Fort said. "They will pay top dollar for the best they can buy."

Many U.S. railroads are trying to unload their unneeded cabooses, but Fort said the Saudis were not interested in refurbishing castoffs. "The Saudis have a lot of money . . . and there are few U.S. cabooses with the creature comforts and conveniences they want," he said.

Building the cabooses from scratch is the second shift in direction for C&S. Originally, the company's founders thought they would repair rail cars. But business was slow, so they switched tracks to enter the growing market for restoring mass-transit passenger cars.

The company takes old subway cars and does cleaning, electrical rewiring, valve repairs, wheel and axle work and welding, as needed, to restore the vehicles to the original manufacturer's specifications. Refurbishing a subway car can add five to 10 years to its life -- a 50 percent extension -- for about $250,000; in contrast, it costs about $2 million to buy a new car, Fort said.

"Most of the old cities," including Boston, Philadelphia and Chicago, have aging metro systems that will need new or refurbished cars in the next few years, Fort said, adding that "80 percent of the business is in New York City."

New York has more than 5,000 mass-transit cars, and plans to refurbish about 500 cars a year. "Our market will go as New York City goes," Fort said.

He estimated the total market at more than $200 million a year "and growing." C&S hopes for a share of that market worth about $3 million to $5 million a year. Its major competition is from heavyweights such as General Electric Co. and Morrison-Knudsen Co. Inc., both of which have annual revenue in the billions.

Although mass-transit cars now account for most of C&S's revenue, its rail-car repair business has gathered steam. Clients include Amtrak, the Southeastern Pennsylvania Transit Authority, private companies such as E. I. du Pont de Nemours & Co., and railroad giants such as Norfolk Southern Corp. and CSX Corp. C&S also has worked on tanks for the Department of Defense.

In four years, C&S has grown from about 25 employes to about 150 and expects revenue of $10 million in 1985. The privately held company "is close to break-even," Fort said. "The profit performance is attributable to the start-up costs of getting into new markets. You can't just put a shingle out and say, 'I build mass-transit cars.' "

The company's strategy has been to "bid competitively, take on new jobs and prove we can do them," to build up a reputation and client base, Fort said.

C&S "will not make a lot of money" on the $800,000 caboose contract, but Fort sees the deal as a way for the young company to prove itself to potential Saudi customers. "We have to get known, and the only way to get known is to ship them a product."