An old forgotten railway that rolls across Mexico's narrowest neck of land is about to turn into a busy path between the seas designed to lure traffic from the Panama Canal.

Within three months, when U.S.-built trains start hauling containers from coast to coast in less than a day, this land bridge will both be a very old dream come true and a symbol of Mexico's hopes to change rapidly from an agricultural nation into a modern industrial one.

Besides servicing Mexico's own fast-growing petrochemical and other industries spinning off the oil boom, the new corridor aims to cash in on the container trade flow between the Far East and Europe as well as traffic between the U.S. west and east coasts, much of which now passes through the canal.

The Mexican government agency organizing the rail service says that ships, by not sailing as far south as Panama and avoiding the waiting lines there, can save up to five days by using the Mexican route. The sea distance between New York and San Francisco, for example, is reduced by more than 2,000 miles.

On both sides of the Isthmus of Tehuantepec, at the ports of Salina Cruz on the Pacific and Coatzacoalcos on the Gulf of Mexico, construction gear is still chipping away at new container terminals.

Giant cranes are already in place to pick containers off the ships, put them on trains and lift them back on board again on the opposite coast. The old 175-mile, one-way railroad has new sleepers and 22 bypass tracks. The government says it has invested almost $100 million and expects to recover it within five years.

Although the swampy, hot route across the isthmus is relatively dull, for centuries this narrowest point of the North American mainland has caught the imagination of dreamers, entrepreneurs and military planners wanting to connect the Atlantic and the Pacific.

The Spanish conquerors hauled their cargo across Mexico not far from here. They unloaded their galleons coming from the Philippines at Acapulco and took their spoils to Veracruz on their way to Spain. Long before the Panama Canal construction began, the isthmus with its swollen rivers was seriously considered as the site for an interoceanic canal. One plan even involved pulling ships along a broad-gauge railway right across the land.

A British entrepreneur, Lord Cowdry, laid the railroad in 1907 and it proved to be a profitable enterprise. Yet the line's period of glory was short; seven years later the canal opened, and Mexico was in the middle of revolution.

During World War II, the U.S. military built airports on the isthmus and carved a highway across the Sierra Madre mountains. This road is coming in handy today.

Running parallel to the rails and recently overhauled, it can accommodate trucks that are to ferry refrigerated containers across the isthmus in six hours, half the time it takes by train. Both road and train traffic will end at vast new docking areas, where Mexico is also building container storage and repair sheds.

But for all its modern approach, the Mexican scheme is not likely to pose a serious business threat to the aged Panama Canal, shipping experts say. Mexico plans to move 70,000 containers in the last months of this year, while the Panama Canal handles that many in three days.

In the following years, however, the Mexicans hope to move 500,000 containers per year, roughly equivalent to the load of 500 large ships.

Fernando Bueno Alvarez, who heads the government agency running the land bridge, said: "We're really offering a third option. To go from west to east, say, you go either across the U.S. by train, which is the fastest and the most expensive; you go through Panama, which is the cheapest but the slowest. Or you come to Mexico. It's a matter of price or time, whichever is most convenient."

The competitiveness of the Mexican route, shippers say, will depend largely on the speed with which the unloading, crossing and loading can take place. Ship to ship, they reckon the whole operation will take three days. "If it works smoothly it can save a big shipping company now going through Panama one ship per year," an American said.

While Panama charges tolls based on tonnage, Mexico's rates will be based on container size. "A 20-foot box will cost $430 ship to ship, and a 40-foot one will cost $620," said Bueno Alvarez.

Mexican officials who have promoted their new route among the world's trading nations say they have found a great deal of interest, particularly in West Germany, Australia, Sweden and France. Japan, the second largest user of the Panama Canal, is already said to be reconsidering some of its routes.

But Mexico is clearly also out to cash in on the new facilities to relieve its own heavily clogged and antiquated port system. Backlogs of industrial supplies and foodstuffs have caused dramatic losses in the past few years of Mexico's oil-financed industrial boom.

Its shortage of transport facilities prevents it from diversifying its foreign markets. Against stated policy, it is increasing its U.S. trade over relatively short truck or train routes.

Coatzacoalcos, Mexico's principal oil port, is greatly overcrowded but is to get relief from the new installations at the container docks.

The land bridge, the government says, will also bring a great economic boost to Salina Cruz and the southern isthmus, where almost 1 million people live in poverty despite the area's potential wealth. Salina Cruz has been chosen as one of four industrial "development poles" away from the large cities, to spread opportunity and slow the growth of Mexico's urban conglomerates.

Coatzacoalcos, another of the four, is already feeling the impact of rapid development: in five years its population has multiplied five times. "Salina Cruz is next," a government official said. "The region around it has wood, minerals, natural gas and cheap hydroelectric energy. It's virtually untapped. Now it will grow fast."