High-speed US rail network: feasible, or science fiction?

They call it ''Rail America,'' and it is probably more ambitious than the Interstate Highway System, that is now approaching completion after 27 years of work and more than $80 billion.

But Northrop Services Inc., the think tank that conceived Rail America, says that without it, the nation's transportation system will slow to a crawl by the turn of the century.

With Rail America, Northrop planners say, the nation's economy would be reignited, freight and passengers would have a high-speed alternative for reaching their destinations, and the US transportation system would run far more efficiently.

That's a tall order, and some planners see it as science fiction. They say the proposal is too vast to be feasible. Regional high-speed rail systems are fine, these planners say, but a coast-to-coast system is unnecessary and too expensive to pay for itself.

But Northrop engineers say the system could work with technology already developed and waiting on the shelf to be used. They know the plan must be validated before it can be considered seriously.

They hope that the wave of interest in high-speed rail, with systems being planned in at least 11 states, will move political, social, and business opinion in their direction.

What they envision is a 21,000-mile grid system of high-speed rail tracks that would crisscross the United States north and south, and east and west. Riding those rails would be the latest generation of electric-powered trains that could travel at speeds at least 160 miles per hour or perhaps more than 250 m.p.h. The system would take 20 to 30 years to build, and would cost - very roughly - $10 billion a year.

The trains would carry passengers and light cargo - no coal or phosphate. Below the tracks in underground passageways would be a right-of-way open for anything from transcontinental electronic mail service to water pipes.

Having an integrated service of freight, passengers, and underground right-of-way would make the system attractive to private investment, Northrop executives say. It would have more chance of being profitable than a system that catered to just one of those transportation needs.

Cargo that takes at least 12 days to cross the country now on conventional rail lines could arrive in two days on Rail America, Northrop predicts. And people wanting to travel 200 or 300 miles between cities would not have to drive their own cars or take a short hop in an airplane. They would catch the next train and arrive within two hours.

Trucks and cars are inefficient for trips of more than 250 miles, says James Kovach, a Northrop program development manager. And airplanes waste fuel and time on trips of less than 1,000 miles. Many highways have reached or exceeded the capacity for which they were built, he adds, and many airports will be saturated by the end of the century.

When Northrop engineers talk of railroads, they are not referring to a patch-up job of the nation's existing railroads that have been deteriorating for more than 30 years.

''We have to address this as a new mode of transportation,'' Mr. Kovach says. ''The current railroads are freight services, and you can't run high-speed trains and heavy freight trains on the same tracks.''

Northrop is suggesting an entirely new track system that would largely follow existing highway, railroad, pipeline, or power line rights-of-way.

Its engineers are talking about putting on those tracks the latest in rail technology, be it 185-m.p.h. trains, such as the French have recently completed, or 250-m.p.h. magnetic-levitation trains that now run on experimental tracks in West Germany and Japan.

Such a new system is needed, Northrop planners say, because America's present transportation network is inefficient and is getting so crowded that it will have a hard time meeting the nation's needs within the next two decades.

''If we do not already, we will in the near future have our economic growth constrained by our transportation system,'' said James Harp, Northrop's services manager of program development, at a congressional subcommittee examining advanced rail technology.

All other plans for improving the nation's transportation system are simply patch jobs, Northrop planners say, that will do little to provide for growth.

Kovach says Northrop has had trouble finding a constituency to back its plan, but after three years of peddling the idea in Washington, it's beginning to get a serious hearing.

''We think their idea is important to consider,'' says Curt Stanford, a transportation analyst for the House Committee on Science and Technology. ''The transportation subcommittee did recommend that Congress authorize a concept validation analysis to test the feasibility of the system.''

Northrop would like about $10 million from the federal government to test its concept during the next two years.

Northrop has to overcome a prejudice against railroads that has grown up with Amtrak's massive federal subsidies, Kovach says.

''The main problem is that most people have Amtrak on the mind,'' he adds. ''It's hard to get past that and focus on our total transportation problem. We would like to have people think of it as an Interstate highway system rather than Amtrak.''