The Pentagon's latest "race track" scheme for basing a new MX supermissile involves a Rube Goldberg collection of machinery and gimmickry to make the rocket concealable and movable yet still detectable by Soviet spy satellites.

The $30 billion system (at least that is the Pentagon's latest cost estimate) will involve a heavy-duty, four-track railroad line, a 210-foot-long box to provide camouflage for the rocket and its truck-transporter and huge concrete barriers that will be constructed and destroyed deliberately to facilitate Soviet monitoring of what is going on.

As described in an authoritative Pentagon document made available to The Washington Post, this is how the new system -- now the one preferred in the Pentagon -- will operate:

A series of valleys in the deserts of Nevada and Utah will be paved with 200 separate "race tracks, or roughly oval paved roadways, each with 23 separate spur roads that look like driveways in a housing development. Each spur will lead to a concrete-and-steel shelter strong enough to withstand all but a nearly direct hit by a Soviet nuclear weapon. The shelters will be about 1 1/2 miles apart. Each race track will be about seven miles across and two or three miles wide.

There will be just one MX missile for each of the 200 race tracks. The idea is to move the missile from one shelter to another at random, in ways the Soviets cannot detect, to fool Soviet targeters who might contemplate a strike to try to wipe out the entire MX force.

As the system is now designed, 200 misiles will be shuttled among 4,600 shelters. To attack those shelters with any confidence, U.S. planners reckon, the Soviets would have to aim two extremely accurate missile warheads at each shelter, for a total of 9,200. This is more than the Soviets would have under SALT II, though without the treaty they could acquire that many or more.

The Pentagon calculates that it could add more shelters if the Soviets and more warheads.

The system is intended to provide invulnerable land-based rockets, but it is also meant to be "verifiable" under SALT III, if there is such a treaty. (The Mx will not be deployed until at least 1986, a year after SALT II would expire.) This forces the Pentagon to design complicated features into the race track scheme to reassure the Soviets that there really is only one misile per race track.

To begin with, the missile would be assembled in a "designated assembly area" in facilities that are as open as possible, perhaps with removable roofs. It will take at least a week to assemble each missile, and only one will be put together at a time.

At the same facility, the "transporter Erector-Launcher" that will carry the missile around the race track from shelter to shelter will aslo be largely -- but not entirely -- assembled.

These two items are too heavy and big to be moved by conventional transport. They can only be moved on a specially constructed, four-rail road that connects the assembly area to each of the 100 or so valleys that will be the locations of the 200 race tracks.

Each rocket will weigh 95 tons. The transporter-erector-launcher, a huge flatbed platform pulled by a truck from which the rocket can be launched, will be nearly 200 feet long.

These giant components will move slowly along the four-rail track to a spur connecting the main line to each race track valley. Each spur will be blocked by a huge barrier of concrete, or perhaps a long section of track will be missing. To deliver the rocket, then, the barrier will have to be destroyed, or the missing track laid. Once the rocket is into the spur, the barrier will be restored.

The point of all this is to allow Soviet spy satellites to watch the entire process, and to see that extra rockets connot be seaked into the race track.

Once on the spur the rocket will be taken to a final assembly point, but reaching it will require passing through a second huge barrier like the first.

At the final asembly point the rocket and its transporter-erector-launcher are put together and driven onto the race track. The vehicle will carry the rocket from shelter to shelter, resting for brief periods inside one of them, then moving on.

This procedure must be hidden, so another machine is required. This is a "visibility shield," a box-lide vehicle that can cover the entire rocket and launcher as it moves from shelter to shelter. This shield will perform decoy maneuvers simulating real movements by the rocket and launcher, so that Soviet satellites will bot know where the rocket really is.

Periodically, however, they will be shown where it is. All the shelters will have "plugs" in their roofs that are removable by large cranes. At specific intervals the plugs will be removed, allowing a view inside from space. When the plugs are removed from all 23 shelters on one race track, the Soviets should be able to see that only one of the 23 contain a missile.

The rocket transporter will have the capacity to "dash" from one shelter to another after a Soviet attack has begun. The Pentagon wants to design the transporter so it can dash without a driver -- by remote control. Also, the rocket can be launched from anywhere on the roadway -- it does not have to be inside a shelter.

In normal times the race track roadways will be open to the public, a feature that apparently appeals to the governors of Utah and Nevada. The Pentagon, with an eye on environmntal concerns, hopes to provide electric power for each shelter with solar energy. CAPTION: Map, Racetrack Basing Scheme for the MX Missile, By Richard Furno and Dave Cook -- The Washington Post