There are, of course, a finite number of railroad ties in the new section of Metro's Orange Line from Ballston west to Vienna.

But lying there in the median of I-66 and viewed at 55 mph, the two streams of perfectly aligned dark brown timbers flow and dip and curve as the highway does, rippling by in a way that suggests an infinitude.

The subway won't be running here until the spring of 1986, but the $7 million worth of track already is being laid in a chain-link fence enclosed trough about 35 feet wide.

The tracks are railroad--not subway--to the 40 men who are taking Metro 10 miles from Glebe Road in Arlington to Nutley Road near Vienna, and working on the railroad there is much the same as it was a century ago.

"There is a lot of art associated with this," says Michael Ryan, waving to a tangle of rails, ties, switches and sweating workers who seem to be shoving, heaving and grunting far too much to be making art.

Ryan is Metro's resident engineer on the track laying, and yesterday morning he was in the middle of the interstate, patiently explaining the art of railroading.

The art is this: When the two strings of steel track are finally pinned to the wooden ties with stakes, they must be 56 1/4 inches from each other, give or take no more than an eighth of an inch, for mile after mile, up hills and around curves.

In the jargon of railroading, the distance between rails is the gauge, the amount of give permitted in the gauge is called the tolerance.

Today's subways are, as railroads go, pretty intolerant. The smaller the tolerance, the smoother the ride, and Metro is striving for smoothness. In comparison, the 56 1/4-inch gauge that freight and passenger trains use on the line from Washington to Brunswick, Maine, has a tolerance five times as lenient as Metro.

"Normally, we'll put one rail on the money, and gauge it to the other side," explains Bill Blaine, who is project manager for Atlas Railroad Construction Co., the Pennsylvania firm which is laying the Orange Line. Nowadays, Blaine says, the opposite rail is pulled into place, and a machine called a hydrospiker comes along and sets the gauge between the rails, spiking the second rail into position.

Blaine, 37, has been laying rail for 15 years, abandoning highway construction, partly "because originally there was a mystique here, with the clackety-clack of the rails." That is gone now, silenced by the modern technology of continuous welded rail, which eliminates noisy track joints.

Technology has not eliminated the brute force required to build a railroad. Hardwood railroad ties, 8 1/2 feet long, 7 inches wide, 9 inches high, weigh 250 pounds each. Steel rail, which, for the Orange Line, has come from France in 60-foot pieces, has been welded into 720-foot strings, which weigh 115 pounds a yard, 27,600 pounds a piece. Steel spikes weigh about a pound a piece.

Yesterday morning, a gang of workers spent several hours wrestling a few short pieces of track into precise positions near the West Falls Church station. The gangs are young, says Blaine, average age about 30. And most of the men are experienced track layers who migrate from job to job.

"Most of these guys are from Pennsylvania and West Virginia," said Blaine. "If you want good track layers, that's where you have to go to get 'em."

The steel rails the crews were jacking and jimmying into position are pliable. "They have the same characteristics as spaghetti," said Ryan. "You could all but tie them in knots."

As he spoke, he walked between a curving section of track and a 720-foot piece of rail which snaked along, following the ripples and bumps of the ground as though it were string. It emitted eerie noises, twanging like an oversized guitar string when plucked accidentally by a piece of equipment.

The ties that will bind the Orange Line are being laid on top of eight inches of egg-size granite stones, called ballast.

Once the ties are set in place, a precise 27 inches apart on mainline track, steel base plates are spiked to either end of the ties. Track is laid across the plates and spiked into place.

Then hopper cars riding the newly laid rails wheel in and add another four inches of ballast between the ties.

Next, a many-armed contraption called a tamper rolls along the track, actually lifting up the rails and ties, tamping down the ballast and fine-tuning the gauge as it goes. ("It replaces about 100 workers," said one railroader.)

Finally, the coarse, rusted orange rails are ground to a stainless steel silver by a rail polisher.

Although much of the work is done by machine, and there is little of the ritual and pageantry which once surrounded railroading, the sharp, sour smell of creosote, which coats and preserves the ties, still hangs in the air wherever there is a real railroad.

There will be, says Blaine, no celebration, no golden spike when the track is completed.

There is, however, a certain satisfaction to the work, to laying the rail perfectly. And, 24-year-old track man John Smigovsky says he will "very definitely" bring his children-to-be out someday to see the track he laid out to Vienna. "I'd like to do that, yeah," he says.

And there is, of course, a certain number of ties under those two, 10-mile-long stretches of rail from Ballston to Vienna. Mike Ryan knows exactly how many: 57,150.