The Gyeongui line was separated in 1945 at the end of World War II, when the peninsula was liberated from Japanese colonial rule but also divided between a USA -controlled southern side and a Soviet-controlled north.
"They're watching this hearing", he said.
At a historic summit in Singapore earlier this month, U.S. President Donald Trump and North Korean leader Kim Jong-un agreed to the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the U.S.
U.S. Secretary of State Mike Pompeo, then serving as director of the Central Intelligence Agency, reportedly pushed for Harris' placement in Korea.
"I think his point was a fair one", he added.
Pompeo was also questioned about Trump's saying after the summit with Kim that North Korea no longer posed a nuclear threat, an assertion questioned by analysts and mocked by Democrats.
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But Harris has since toned down his rhetoric in cue with the Trump administration's engagement with the North on denuclearization talks. "For the moment, we have reduced risk", Pompeo told the subcommittee.
On Tuesday, Vice Transport Minister Kim Jeong-ryeol, the South's chief delegate at inter-Korean railway talks, said that full cooperation between South and North Korean railway authorities would depend on progress in negotiations between Pyongyang and Washington on denuclearization.
Trump and Kim signed a joint declaration at a summit in Singapore on June 12 and pledged to work toward peace and to denuclearize the Korean Peninsula.
The Yongbyon Nuclear Scientific Research Center is located about 60 miles north of the country's capital and is a core part of its nuclear weapons program.
South Korea has ambitions to significantly extend the eastern "Donghae" line so that it connects its southernmost port of Busan with North Korea's northernmost industrial cities of Chongjin and Rajin.
"We should remain skeptical that North Korea's nuclear calculus has changed dramatically".