South Korea has a new and efficient high speed service known as Korea Train Express, abbreviated KTX. It is operated by national train operator, Korail.
Most KTX trains leave from Seoul Station or Yongsan Station and terminate in the cities of Busan, Gwangju, Mokpo, Yeosu, Pohang, Gangneung and Jinju. Most major cities are served in between.
The KTX train is regarded as an easier, more comfortable and cheaper way to get around South Korea than by plane. If you figure in security and getting to/from the airport it may also just be faster to go by train. Ticket prices are slightly below the equivalent airfare of the major airlines. The weekday Standard class price for Seoul-Busan is around ₩57,000.
There are two types of KTX seat classes; First and Standard. First class has wider seats, free snacks and bottled water.
There are three types of KTX trains, KTX-I, KTX-Sancheon, and KTX-Eum. KTX-Sancheon has better seats and power plugs for each seat. KTX-I trains have fewer power plugs between the window. There is no extra fare for KTX-Sancheon. KTX-Eum is the latest train, and runs only on the Jungang line to Wonju, Jecheon, Yeongju, and Andong.
The KTX service does not connect to any other country, although many Koreans would hope that one day it could run through North Korea and onto the rest of the Asian continent. Plans by South Korea and China to connect their networks through North Korea are unlikely to ever be fulfilled in light of the North Korean political situation.
Also, separate company, named SR, operates high-speed service SRT from Suseo station, located at southeastern Seoul. It serves the same route as the Gyeongbu and Honam KTX after Cheonan-asan. It uses separate booking system and app, but has same KTX-Sancheon train and its offline tickets can be bought from the Korail counters, just like the regular KTX. It is slightly cheaper and more convenient when you're going to Gangnam district in Seoul. Most of the services by Korail such as the Korail pass will not be applied. SRT has a sparser schedule than Korail with fewer departures.
All lines start in Seoul and terminate in either the southwest or southeast of the country. Seoul has three main KTX stations (Seoul, Yongsan, Cheongnyangni); Seoul station for mainly Gyeongbu, Gyeongjeon lines, and Yongsan station for Honam and Jeolla lines, and Cheongnyangni for Gangneung line. Also, Suseo station at the southeast of the Seoul serves SRT.
Keep in mind that few KTX trains use conventional line to serve stations at Suwon, Gupo (West of Busan), Nonsan, etc. These trains are slower, but cheaper.
These are the its major stops.
Some KTX trains proceed north from Seoul to Haengshin.
There were KTX services from Incheon International Airport in the past, but was discontinued after 2018 Olympics. If you're going to use KTX (except those using Gangneung or Jungang KTX) from the airport, ride 6770 bus to get to the Gwangmeyong Station and continue on the KTX. It's faster and cheaper than going all the way to Seoul and riding the KTX.
Also, when you're going to Incheon Airport, you can use City-Airport Terminal at Gwangmyeong station. You can go through check-in and immigration if you're using selected airlines.
A plan to build an undersea tunnel to Jeju has been mooted by the government, with a KTX service taking 2 hours and 40 minutes to reach there from Seoul. The Seoul-to-Jeju airline route is the busiest in the world, although construction would involve building the longest tunnel in the world and it is not clear if Korea would be prepared to invest the huge amount needed.
Tickets may be purchased up to one month in advance at any Korean Rail station ticket office, or online at Korail's website. The online option has the benefit of allowing you to book in English, and tickets can be picked up in advance from any Korean Rail Ticket Office. However, you must bring your passport for identification when picking up the tickets.
Standby (입석, ipseok) tickets cannot be purchased online; those can only be purchased at ticket machines at the major stations.
Residents in Korea have the option of purchasing tickets from the Korail Talk app (English, Chinese, and Japanese are available, but has some errors). A Korean credit card is required.
SRT trains can be booked from separate website (dead link: January 2023).
During Lunar New Year (late Jan or Feb) and Chuseok (Sep or early Oct) dates, it is practically impossible to travel by train during these periods due to all the crowds.