Korea's six largest cities all have subway systems and all are connected by the Korean railroad system, Korail, with service every 15 to 60 minutes.
Here are two buildings at one of the exits at the COEX underground shopping in the Gangnam district, said to be Asia's largest, although currently undergoing remodeling.
Take another exit and these buildings would not even be within sight.
In the winter, seats on the subway are automatically warmed.
The subway is a railroad, of sorts, most of it underground. The subway is connected to the larger Korean railroad system is extensive, connecting all major cities. Coming back from Japan I chose to go back to Busan, on the southern coast, rather than fly from Osaka to Seoul in order to ride the Korean "bullet train" because I love trains and to see more of the countryside. It runs about every half hour. Called KTX, it uses the same technology as France's famed TJV system. It was opened in 2004 and improved in 2010 to gain speed.
The KTX did indeed rock along at 180 miles an hour and get there is less than three hours but made six or eight stops which somewhat took away the thrill of the ride. No doubt some trains go faster. As with the French system, the ride is so smooth and quiet it's hard to believe you're going as fast as you are.