Personal comments: I have been on quite a few subway systems in different countries and continents, and the Seoul version has to be one of the best, maybe even THE best.

• It is very comprehensive; one can get to almost anywhere in the city and to the two airports easily and fast.
• Trains are very frequent, clean and comfortable, with AC and so on - and wi-fi in many places.
• Almost ALL stations have full-height platform screens. London has NO screens, while Tokyo/Yokohama have HALF-screens, but in Seoul you cannot jump in front of a train.
• Seating is ALONG THE SIDES of the carriages as in Japan, but not London.
• Navigation is easy, and messages are given in English as well as Korean.
• My impression is that the Japanese subway can get more crowded. I was almost always able to get a seat in Seoul, but not in Japan.
• Ticket cost is very reasonable, FAR more so than in London for example.
• You can get a "T-Money" card which enables fast payment at the gates, but this card can also be used on buses and for TAXIS, too. And when you have finished with it, you can get a REFUND of any money still on it.
• WI-FI? READ THIS!

Other features: Some stations are VERY long, with a platform at either END of a very long tunnel with little shops alone either side. My impression is that Tokyo/Yokohama hub-stations complexes are in general larger, with HUGE underground shopping complexes. However, Seoul also has a lot of shops underground.

FROM WIKIPEDIA: The Seoul Metropolitan Subway is a metropolitan railway system consisting of 22 rapid transit, light metro, commuter rail and people-mover lines. The system serves most of the Seoul Metropolitan Area including the Incheon metropolis and satellite cities in Gyeonggi province. Some regional lines in the network stretch out to rural areas in northern Chungnam province and western Gangwon province, that lie over 100 km away from the capital, as well as Suwon.

The network consists of numbered lines 1–9, which serve Seoul City proper and its surroundings and other specifically named regional railways that serve the greater metropolitan region and beyond. Most of the system is operated by three companies – Seoul Metro, Korail (Korea National Railroad) and Metro 9. However, there are several other lines stretching out to regional provinces.

History: Its first metro line, Line 1, started construction in 1971 and opened in 1974, with through-operation to Korail suburban railways. Today, the network is one of the largest and most efficient urban railway systems in the world, with 331.5 km (206.0 mi) of track on lines 1–9 alone; wireless and internet service on all trains; and platform screen doors at almost all stations.

Under the Japanese ODA loans, the first line of the Seoul Subway network started construction in 1971. The first section of subway was built using the cheaper cut-and-cover construction method. Line 1 opened in 1974 with through services joining surrounding Korail suburban railway lines similar to the Tokyo subway. Today, many of the Seoul Metropolitan Subway's lines are operated by Korail, South Korea's national passenger and freight railway operator. This is similar to Europe and Japan, where the national railroad often operates local mainline urban railways, such as the S-Bahns in Germany, operated by subsidiaries of Deutsche Bahn, or JR East in Japan, which operates many other urban rail systems in Japanese cities.

Technology: All directional signs in the system are written in Korean, English and Hanja. In trains there are in addition many LCD screens giving service announcements, upcoming stop names, YTN news, stock prices and animated shorts. There are also pre-recorded voice announcements that give the upcoming station, any possible line transfer, and the exiting side in Korean, followed by English. At major stations, this is followed by Japanese, then Mandarin Chinese, as well. Seoul Subway uses full-color LCD screens at all stations to display real-time subway arrival times, which are also available on apps for smartphones. Most trains have digital TV screens, and all of them have air conditioning and climate-controlled seats installed that are automatically heated in the winter. In 2014, it became the world's first metro operator to use transparent displays for ads when it installed 48 transparent displays on major stations of Line 2 in Gangnam District. All lines use the T-money smart payment system using RFID and NFC technology for automatic payment by T-money smart cards, smartphones, or credit cards and one can transfer to any of the other line within the system for free.

It has been described as the world's longest multi-operator metro system by route length. The system was rated as one of the world's best subway systems by CNN and Jalopnik. It is notable for its cleanliness and ease of use along with advanced technology such as 4G LTE, WiFi, DMB, and WiBro accessible in all stations and trains.Nearly all stations have platform screen doors installed; only Gaewha and some minor Korail-operated stations remain with open platforms. By 2017, Korail will completely install screen doors in every station and platform. The world's first virtual mart for smartphone users opened at Seolleung station in 2011.

Lines and branches: The system is organised such that numbered lines, with some exceptions, are considered as urban rapid transit lines located within the Seoul National Capital Area (SNCA), whereas wide-area commuter lines operated by Korail provide a metro-like commuter rail service that usually extends far beyond the boundaries of the SNCA, rather similar to the RER in Paris. The AREX is an airport rail link that links Incheon International Airport and Gimpo Airport to central Seoul, and offers both express service directly to Incheon International Airport and all-stop commuter service for people living along the vicinity of the line. While operating hours may vary depending on the line in question, the Seoul Metropolitan Subway generally operates from 5.30 am until 1 am on weekdays, and from 5.30 am until midnight on weekends.