MUNSAN, South Korea, May 17 — Trains crossed the border between North and South Korea on Thursday for the first time in 56 years, an event both sides described as a milestone for reconciliation on the divided Korean Peninsula.

As white balloons soared into a blue sky, soldiers swung open gates topped with barbed wire shortly after noon to let the five cars of the South Korean train cross the 2.5-mile-wide demilitarized zone, the world’s most heavily armed border, and enter North Korea.

At the same time, about 144 miles east, a North Korean train trundled down the coast.

Although these were technically just one-time test runs on two short stretches of railroad that were linked through the demilitarized zone several years ago, their symbolic importance was unmistakable. The last trains that crossed the border carried refugees and wounded soldiers to South Korea from the North during the Korean War in 1951.

Photographs of the bullet-scarred, rusting hulks of wartime locomotives trapped in the demilitarized zone have symbolized a divided Korea and a conflict that has never been formally resolved, because the war ended in a truce, not a treaty.

“These are not just test runs,” said South Korea’s unification minister, Lee Jae-joung. “They mean reconnecting the severed bloodline of the Korean nation.” He spoke during a ceremony at Munsan Station, about seven miles south of the demilitarized zone. “The trains carry our dream of peace.”

His North Korean counterpart, Kwon Ho-ung, who was also in Munsan, said the trains represented the “Korean nation’s wish to gallop to the destination of reunification,” despite what he called outside forces — apparently a reference to the United States — that are “not happy with reconciliation among Koreans.”

The South Korean train, carrying 150 people from both sides of the border, pulled out of Munsan around 11:30 a.m. as fireworks exploded overhead. It traveled about 15 miles to Kaesong, a North Korean border town where South Korea runs factories employing less-costly labor from the North. The North Korean train had a similar journey, from the Mount Kumgang resort to Jejin, 15 miles to the south.

South Korea has long dreamed of building a trans-Korea railroad that would connect its trains to China and to the Trans-Siberian Railway. A route through North Korea would provide overland access to the rest of Asia.

A trans-Korea railroad would offer a faster and cheaper way for South Korea to send exports that are now shipped by sea to China and Europe. It would also provide a shortcut for Russian oil and other natural resources transported to South Korea. Such a rail system would save South Korea $34 to $50 a ton in shipping costs, said Lim Jae-kyung, a researcher at the Korea Transport Institute.

But creating such a system, transportation analysts and government officials say, would require years of confidence-building talks and billions of dollars in investment in North Korea’s decrepit rail system.

Officials acknowledge that North Korea will probably have to give up its nuclear weapons and improve its human rights record before it could attract significant investment from South Korea or international development aid. Six-nation talks aimed at ending North Korea’s nuclear weapons programs have been stalled for months.

“I cannot understand why we should give rice, flour, fertilizer and everything else the North Koreans want when they don’t do anything for us,” said Hong Moo-sun, 71, one of a dozen South Koreans protesting just outside Munsan Station on Thursday.

The protesters were calling for North Korea to return their relatives, among the hundreds of people taken to North Korea after the war and believed to be held there against their will.

Members of the Grand National Party, part of the conservative opposition, called the event on Thursday a “train of illusion” organized to draw voters’ attention in an election year.

South Korean officials say a trans-Korea railroad would invigorate inter-Korean trade, which tripled to $1.35 billion last year from $430 million in 2000. It would also bring cash to North Korea, which could collect an estimated $150 million a year in transit fees from trains that pass through its territory, Mr. Lim, the researcher, said.

But procuring international aid to renovate the rail network and letting trains from one of Asia’s most vibrant economies, carrying exports and tourists, rumble through its isolated territory could threaten the North Korean government, experts say. They say North Korea now relies on keeping its people ignorant of the outside world to maintain its totalitarian grip on power.

Both Koreas agreed in 2000 to reconnect their rail systems, which had been severed by aerial bombing during the war. It took three years to relink the tracks on the west and east ends of the border. After four more years of haggling and delays, the North Korean military agreed this month to allow the one-time test runs.

The agreement came after South Korea promised to send North Korea 400,000 tons of rice, as well as $80 million worth of raw materials for shoes, soap and textiles.

South Korea has spent $589 million on reconnecting the rail system, including $195 million worth of equipment, tracks and other material lent to North Korea.

South Korean policy makers have called for patience in working toward reconciliation with the North. They have often been accused by conservative politicians and civic groups of giving in to North Korea’s strategy of extracting economic aid for every step toward reconciliation.

“This is a precious first step for a 1,000-mile journey,” Mr. Lee, the unification minister, said Thursday.