President Carter has decided to build the new blockbuster land missile called the MX and to make it mobile to decrease its vulnerability to Soviet attack.

Administration officials said last night that the president's decision means that the 190,000-pound missile - 92 inches across at its base and capable of carrying 10 warheads of 335 kilotons each - will advance into the pre-production stage known as fullscale development.

The actual decision reached by the president this week was to proceed with the largest model of the MX under consideration - a model suitable only for deployment on land. Carter and his senior advisers decided to postpone a final choice of basing system for the new missile.

Carter's decision is likely to lead to a system whose missiles deliberately are hidden from Soviet spy satellite - a system that is likely to please hard-liners and upset arms control advocates who fear a new arms race in the field of mobile, land-based ICBMs.

Carter has just about decided to put the MX on railroad cars, officials said, but will not announce that intention until after conferring with politicians and citizen groups in the states involved.

The president's plan calls for using government land in Arizona, New Mexico, Nevada and Utah for separate strips of railroad carrying the huge missiles.

The MX is a new generation intercontinental ballistic missile (ICBM), powerful and accurate enough to blow up Soviet missiles in their underground silos, and is expected to be the first mobile, strategic land missile deployed by the United States.

The MX would be in addition to the 1,000 Minuteman and 54 Titan ICBMs deployed in underground silos. About 200 MX blockbuster could be built and deployed by about 1986.

All told, the railroad deployment of MX would require 200 strips of government land, each 20 miles long, distributed among the four southwestern states. The track would be laid at the bottom of trenches covered with roofs that could be removed periodically to enable Soviet satellites to verify how many missiles were there.

Special locomotives would shuttle the MX missiles from one protected railroad station to another along the rails. The Pentagon figures that there would be between 8,000 to 9,000 of these "stations" for missiles in the MX network.

Administration officials said last night that the MX railway deployment would cost between $30 billion and $32 billion in fiscal 1980 dollars. This is about $7 billion more than the deployment scheme the Air Force submitted to the White House as its first choice. That scheme called for covertly shuttling 200 MX missiles among 4,000 underground silos to foil Soviet gunners.

Carter rejected that Air Force option, officials said, largely on the ground that it would be too difficult for Soviet satellites to verify which of the vertical, underground silos held missiles even when the United States uncovered them for inspection.

Administration officials said that the president may make the case for the MX rail option in a detailed public statement that he hopes will reassure environmentalists and others in the affected states.

To minimize the protests from hunters, cattlemen and others who fear the MX rail system would keep them from using the designated government land, the administration intends to fence off only the ground above the stations holding the missiles. The public could use the land spread over the concrete roof hiding the railroad tracks themselves.

When the United States chose to remove the roofs on the rail system for Soviet satellite inspection about once a year, a bulldozer would scrape off the dirt and then cranes would lift the concrete roofing off the stations and the rest of the railway system.

One of the reasons that the United States is embarking on building a new ICBM is that defense planners concluded that, theoretically at least, the current force of 1,054 land-based ICBMs could virtually be wiped out by a Soviet sneak attack some time in the early 1980s by a relatively small proportion of Soviet land-based missiles.

This creates the problem specialists refer to as "ICBM vulnerability." The mobile MX system is designed to greatly reduce such vulnerability.