The eastern line was restored last month as Pyongyang began making conciliatory gestures towards its neighbour.
It lifted on Tuesday tough restrictions on border crossings by South Koreans visiting the Kaesong joint industrial estate in the North.
The lines are mainly used to co-ordinate north-bound border crossings to Kaesong in the west and to the Mount Kumgang resort in the east, another joint project.
However, officials in South Korea played down a series of conciliatory gestures from North Korea, saying that the communist state is still unwilling to resume talks on giving up its nuclear programme.
Hyun In-taek, the unification minister, noted the North's conciliatory moves in recent weeks after months of hawkish actions, including numerous missile launches and a nuclear test.
"But I believe it was a tactical, not [a] fundamental, change because nothing has changed in its attitude towards six-party talks and the nuclear issue," Hyun told a seminar with ruling party politicians on Wednesday.
After reaching a six-party disarmament deal in 2007, the North quit the forum in April in protest at the UN Security Council's decision to censure its long-range rocket launch that month.
In May it staged its second nuclear test, incurring tough UN sanctions.
The policy change became noticeable in early August when Bill Clinton, the former US president, visited Pyongyang and met Kim Jong-il, the North's leader, who pardoned two American journalists jailed for illegal entry.
The North has since released five detained South Koreans, lifted border restrictions on its neighbour and agreed to allow more reunions for families divided since the Korean War.