SEOUL, South Korea - North and South Korea struck an ambitious deal Friday to launch cross-border train service and construction projects in the impoverished North despite concerns that reconciliation will founder if the South elects a conservative president.

South Korean Prime Minister Han Duck-soo agreed to start promised shipyard construction along with highway and railroad repairs in the North next year. His North Korean counterpart, Kim Yong Il, agreed that the isolated communist country would begin allowing regular cross-border cargo train service next month for the first time in a half century.

The train service, set to start Dec. 11, will be limited to freight running on a 16-mile track to a joint industrial complex in the North's border city of Kaesong.

The inter-Korean railway carries high symbolic importance, and South Korea hopes it will ultimately be linked to Russia's Trans-Siberian railroad, creating an overland route to Europe that would significantly cut delivery times for freight that now requires shipping by sea.

Conservative opposition candidates who take a critical view of the North Korean regime are leading opinion polls ahead of the Dec. 19 presidential vote, however, and analysts raised concerns that the agreement may be not carried out after South Korean President Roh Moo-hyun leaves office in February.

"Implementation is more important than an agreement," said Baek Seung-joo, a North Korea expert at Seoul's state-run Korea Institute for Defense Analyses. "We've had so many cases in the past where agreements were reached, but not kept."

Baek said the deal's fate depends on the South's next president and how the North views that new leader,as well as developments in the international standoff over Pyongyang's nuclear programs is resolved.

In the worst case, this week's agreement could hurt ties between the two counties if a new conservative president tries to revise it, said Yang Moo-jin, a professor at Seoul's University of North Korean Studies.

"The agreement could rather deepen distrust unless carried out," said Yang.

Since their first summit in 2000, the Koreas have been trying to set aside decades of animosity. They have dramatically increased economic ties and the South has become the North's No. 2 trading partner, after China.

Friday's agreement was a fleshed-out version of a wide-ranging accord struck at an October summit between Roh and North Korea leader Kim Jong Il.

Roh expressed satisfaction with Friday's agreement, saying economic cooperation is the "best means" to advance unification.

"Unless we realize an economically equal relationship, the road to unification cannot but be very slow," Roh told the North Korean delegation at a lunch he hosted in the presidential Blue House.

The North Korean leader is firmly committed to carrying out the summit accord, which he has said "should not end up an empty piece of paper," his prime minister told Roh.

The North has also agreed to turn the area around the Koreas' disputed western sea border into a "peace and cooperation zone" next year by establishing a joint fishing ground and a special economic zone on shore nearby. And the North will allow South Koreans to use the Internet and mobile phones inside the Kaesong industrial park. About two dozen South Korean-run factories employ some 20,000 North Koreans in the joint venture industrial zone.

Internet use in North Korea is normally limited to elite officials, part of the regime's policies to prevent ordinary citizens from receiving any information beyond the steady diet of nationalist propaganda that dominates state-controlled media.

Visitors to North Korea are required to hand over mobile phones upon entry, and get them back upon departure.

Other points of Friday's agreement include promotion of cooperation in the farming, resources development and medical sectors, as well as more reunions of families separated between North and South.

The 1950-53 Korean War ended in a truce, not a peace treaty, leaving the two Koreas technically at war. Their relations have warmed significantly since their first summit, in 2000, although reconciliation has been overshadowed by the standoff over the North's nuclear weapons programs.

Pyongyang recently beginning to disable its sole operational nuclear reactor under a deal with the U.S., South Korea, China, Japan and Russia. This week's talks skirted security issues, as defense ministers from the two Koreas are to discuss the subject later this month.