Trains are probably the fastest and most comfortable means of transportation in South Korea. Most of the time they are faster than buses are they cannot get stuck in traffic, and they are also faster than planes in that train stations are usually in downtown areas rather than in suburbs. There are also plenty of train options for traveling within a city or a metropolitan area, such as subways and suburban trains. For tourists however riding trains in South Korea can be overwhelming given the number of train services available, the different operators, and the different booking processes. This post summarizes all you need to know to book and ride trains in South Korea.
Intercity Train Services
There are several types of intercity train service in Korea. From fastest to slowest (and most expensive to cheapest): KTX & SRT, ITX & 새마을, 누리로 & 무궁화. Most services (except the SRT) are run by Korail (코레일), a public company.
The different train services run by Korail (all but SRT) and SR (SRT) (adapted from https://flytoazuresky.tistory.com/602?category=669623)
High-speed Train Service
KTX, for Korea Train eXpress, is the fastest transportation link between most cities in South Korea. Two classes of service are available, first class and economy, with economy class being usually 20% to 30% cheaper than first class. First class offers more legroom than the already excellent legroom in economy, so the higher ticket price may be hard to justify. Standing tickets can also be booked; these tickets do not have assigned seats and require standing if no seats are available. They are not drastically cheaper than economy tickets (5% to 15% cheaper) and are probably best booked when there is no stop between your origin and destination stations (such as 광명역 – 대전역).
KTX tickets can be booked in English on the Korail website. They can also be bought directly at any Korail station, such as 서울역 (Seoul station) or 용산역 (Yongsan station) in Seoul.
SRT is another train service run by a sister company of Korail. While KTX lines originate mostly from 서울역 or 용산역, SRT service originates from 수서역 in 강남구. SRT service map does not have as many stations as the KTX service, but trains travel at similar speeds (수서 - 부산 in 2 hours 25 minutes) for similar prices. SRT may thus be a better option if you are traveling from areas surrounding 강남. A lot of Korean people do not know about SRT since it is a relatively new service (2016), so be aware of this option when booking your train itineraries.
SRT tickets are booked on a separate website from KTX tickets; they can be booked directly on the SRT website (in English) or at SRT stations, such as 수서역 or 부산역. If the station has both KTX and SRT service, make sure to follow signs for SRT tickets and trains when applicable.
Standard Train Service
새마을 (gradually replaced by the ITX 새마을 service) is a train service running at about half the speed of the KTX service, and stopping at more stations on a given line. For example, while a trip from Seoul to Busan on KTX may have only two or three stops in between, the 새마을/ITX 새마을 service for the same route may stop at 8 or more stations. 새마을/ITX 새마을 tickets are less expensive than KTX tickets and its cars are also less comfortable.
무궁화 is the standard train service (Amtrak speeds) that connect most Korean cities by rail. The service is getting gradually phased out and replaced by 누리로 with different, newer trains. It is slower than ITX 새마을 as it stops at even more stations. While 무궁화/누리로 is definitely not a fast option, it is nonetheless one of the cheapest. It is also one of the least comfortable options.
Touristic Train Service
The DMZ line, the O-line and the V-line in the Seoul area are touristic trains going to the DMZ (DMZ line) or offering scenic views (O-line, V-line) for tourists visiting Seoul. Tickets can be booked through the Korail website.
Metropolitan Train Services
Most major cities in Korea have an independently-run metropolitan train service, usually a mix of underground and overground trains, that connects the city center(s) to the suburban areas. These are different from intercity train services in that, in most cases, tickets do not need to be purchased in advance; instead, passengers need to tap their T-money cards onto the card readers before boarding their train. This includes the Seoul Metropolitan Subway, the Busan Metro, and the Daejeon Metro.
On some subway systems, such as the Seoul one, riders need to tap their T-money card when entering and exiting the subway, as the fare is calculated when exiting. If the T-money card does not have enough funds, money can be added using the fare adjustment machines located prior to tapping out. Some lines, such as the 신분당선 (Shinbundang line) are run by different operators and may charge extra for certain trips.
Some suburban train lines, such as the 경춘선, also have an express service, such as the ITX–청춘. The 경춘선 is part of the Seoul subway and can be paid with T-money, while the ITX–청춘 requires a ticket (bookable on the Korail website) as it is considered a regional train (although running on the same tracks).