SEOUL, South Korea — A South Korean train on Friday crossed into North Korea for the first time in a decade, as the two countries began a joint study on renovating the North’s decrepit rail system and linking it to the South’s.

Over the next 18 days, dozens of officials and engineers from both Koreas will live and work on the six-car train, traveling more than 1,600 miles as they study the North’s rail network and consider what it would take to bring it up to international standards. The train rolled north across the countries’ heavily armed border on Friday morning.

The joint study “signals that inter-Korean cooperation is reaching a new level,” said Kim Eui-kyeom, the spokesman for President Moon Jae-in of South Korea.

The study is one of a number of collaborative projects that Mr. Moon has championed to develop closer ties with the North and demonstrate what economic benefits the country could gain from giving up nuclear weapons. When Mr. Moon and North Korea’s leader, Kim Jong-un, met for the first time in April, the South agreed to help rebuild the North’s railroads and highways, which Mr. Kim said were in “embarrassing” shape.