The bemused security guard pointed to the curving subway train that jumps off the lobby wall of the new Metroplex I office building and said, "Yes, that's the train." It was his first week on the job and he was still in shock.

"And that's the Capital Beltway above you," said the guard, Paul McClain, gesturing at the raised ribbon of highway on the ceiling of one of the distinctive new structures that are transforming the landscape of Prince George's County.

"This is one strange building," he added.

Wait until he sees the authentic Amtrak seats to be installed over the pattern of tracks on the floor and the painting of a whooshing Metroliner that architect Arthur Cotton Moore has planned for the rounded lobby wall. Moore chose the decor to fit what county boosters call the ingredients of the current office and commercial building boom: excellent transportation facilities and cheap land.

The 1.2 million square feet of office space under construction or expected to start in 1981 exceeds all construction completed from 1973 to 1978 and represents a 28 percent increase in the total 1980 office space inventory of 4.2 million square feet. Prince George's still ranks a distant fourth in the metropolitan area office building derby this year, with construction permits valued at $23.9 million through June versus $68.4 million for the region's leader, Northern Virginia.

But if the pattern of construction permits holds for the remainder of 1981, Prince George's will more than double last year's six percent share of the area's commercial construction dollars to 15 percent, according to census bureau statistics. And as the new buildings leap into view around the Beltway, planners hope they will draw attention to what the county believes is a wealth of investment opportunities.

Noted Washington architect Arthur Cotton Moore, designer of Metroplex I, said the building is "a shape that's highly visible from a distance." The sculpted, seven-story brick, glass, steel and wood structure has suggested a ram's head to motorists on both Rte. 50 and the Capital Beltway since it opened last year.

"When you're dealing with a suburban condition you want a highway-scaled supershape because everybody's viewing it whizzing along at 55 miles per hour," said the architect, who designed Georgetown's Canal Square and The Foundry.

Moore's 110,000-square-foot "supershape" is a small part of the 62-acre Metro East project in Lanham. In 1977, Shell Oil Company, noting the success of the nearby Washington Business Park, a sprawling low-rise that pioneered the office park concept in Prince George's in 1976, bought the land between the Penn Central Railroad tracks, Rte. 50 and the Beltway for $3.8 million, or $61,000 per acre.

After putting in sewers, water and roads, Shell began selling parcels for development in 1978. That was the year the New Carrolton Metro station opened across the street from the site, complementing the New Carroll ton/Beltway Amtrak station, which also serves the property. Shell recently sold 11 acres of the valuable tract for $1.6 million, or $145,000 per acre.

"It's purely a matter of location and economics," said Randy MacCuaig of Rouse Associates, a Philadelphia-based development firm that got into Metro East early and has also developed property in Northern Virgina. "Prince George's has the advantage of being located outside of Washington on the major traffic modes with good access to Baltimore. If you're going to tackle both markets, which are not the same, where else do you go?

"Prime space in Washington is $35 (per square foot to lease). Fifteen minutes away in Prince George's you're looking at $15. There's also a greater availability of land," said MacCuaig. "The downside is, can you get the tenants out here?"

Indeed, convincing companies that Prince George's meant business was difficult. Developers did not believe tenants would come from other parts of the region to the county of dowdy repute until someone else did it first, according to economist George Smith of the Prince George's economic development office. Smith said that Washington Business Park, located just across the Beltway from Metro East, with access from George Palmer Highway (Rte. 704), was the catalyst.

"The success of that project figured largely in Shell's purchase of Metro East," said Smith, who has nursed along major county developments for the last 12 years. "That has really been the key to the success of other buildings. . . . Completed projects have been the driving force."

David LaCivita, managing attorney for the branch office of a large Washington law firm now located in Metro 400, a silver-skinned, four-story structure in Metro East, says the building is a success in hours of sleep saved alone.

To beat the traffic when he worked in downtown Washington, he left his Potomac home by 7:30 every morning and stayed at work past 6:30 every evening. Now he leaves home at 8:30 and while west-bound traffic is caught "in the teeth of the grizzly bear, I'm cruising the opposite way," he said.

"Obviously there's a lot more activity and life downtown. You can shop, be with the crowd and feel the activity of the nation's capital. There's no night life out here. After working hours the place closes down," said LaCivita. "But overall the facility is excellent and our clients are a lot happier seeing me out here. It's quiet too. I seem to be able to concentrate better and get more work done.

"I do get to see some trees too," the lawyer added, looking out the large window of his plush office at some of the trees spaced between the growing number of buildings.

Economist Smith and his boss, County Executive Lawrence J. Hogan, hope the county's tax collections will grow by about $890,000 next year based on $60 million in new development over the last two years.

Smith cautioned that the impact on the overall tax base at this point would be slight. The real gain, he said, would be in jobs. He estimated that every 1,000 square feet of office space yields a total of four to five jobs, one of which is likely to be a net new job to the county.

Metro East is perhaps the brightest gem in the Beltway diadem of development, with 363,000 square feet of office space in place and another 310,000 in progress. This does not include the 300-room Holiday Inn planned by developer Mardeck, Ltd. or a possible convention center on the site.

The other major developments in the county, ranked in order of the most imminent construction and immediate impact on the county economy, according to Smith, are:

Washington Business Park -- The complex has 16 buildings on a 225-acre site, with nearly all the space leased to companies including Alcoa, Hitachi Industries, Sony, Sperry Rand and General Electric. About 1,300 people are employed in the complex now, with projections that the number of jobs will reach 2,000 when the remaining 147 acres are developed.

Capitol Office Park -- Two identical eight-story buildings are prominent on the outer loop of the Beltway just west of Kenilworth Avenue. A third is under construction and a 290-room Hilton Inn is planned.

Greenbelt East -- The sleek, 16-story Maryland Trade Center has just been completed. The "for lease" sign of Prince George's tallest building is visible from both the Beltway and the Baltimore-Washington Parkway. An adjoining 34-acre site is being considered for commercial development.

The Golden Triangle -- Named for its location in a perfect triangle between Greenbelt Road, Kenilworth Avenue and the Beltway, the 42-acre site is home to one of the largest Cadillac dealerships on the east coast. Prudential Insurance Company purchased the remaining 37 acres in 1979 for $2.4 million. The firm expects its first seven-story office building to be ready for tenants next March, and a second building of similar scale is being planned.

Calverton Office Park -- Located across the street from the Powder Mill Road and I-95 interchange, the 62-acre site already has a 165-room Ramada Inn complete with ballroom seating for 600. A six-story office building is scheduled to open next month.

Greater Laurel Professional Park -- The 40-acre commercially zoned site is directly opposite the Greater Laurel-Beltsville Hospital and currently has a 40,000-square-foot office building leased to primarily medical tenants. The county has approved industrial revenue bond funding for a 120-bed nursing home, with construction planned to start later this year.

Laurel Employment Area -- The 120-acre tract is zoned for a mixture of industrial and commercial uses. The United Parcel Service has applied for a permit to build a 250,000-square-foot facility there.

Inglewood Business Center -- McCormick Properties plans to build 20 to 40 buildings on this 228-acre site directly across Arena Drive from the Capital Centre. The structures will have a total 3.1 million square feet of floor space and will include a combined hotel and convention center; commercial office space; and research, manufacturing and other industrial space. The entire project is expected to take 5 to 10 years to complete and to provide work space for 9,000 job holders. McCormick is noted for its successful Hunt Valley development in Baltimore.

Auth Road Center -- The 50-acre site is zoned for a planned industrial park. The Group Health Associates Building and the headquarters of the National Association of Operating Engineers have been there since 1972 and 1975, respectively.