The extension would not interfere with the other major river-crossing project that has been championed since the demise of the ARC project: Amtrak's Gateway project, which would add new tunnels to connect the northeast corridor lines in New Jersey to Penn Station in Manhattan.

WSJ: N.J. Subway Idea Lives

Crossing downtown Seoul is the Cheonggye stream -- a river that was covered by a road until it was unearthed in 2005 and transformed into one of the city center's most popular urban oases and worth a stroll in the evening.

CNN: Look into your Seoul for a hit of Korean flavor