Trains are run by the State Railway of Thailand.
Railways converge on Bangkok from all parts of the country: Chiang Mai to the north, Nong Khai on the Laos border, Ubon Ratchathani to the east, Aranyaprathet on the Cambodia border, Sattahip on the coast to the south, Nam Tok to the west, and Padang Besar on the Malaysia border.
Tickets for travel on any trains departing Bangkok may be purchased from any of the larger train stations, as well as online from the official SRT ticketing website D-Ticket or various resellers such as 12go. Bangkok is the only place in Thailand where travellers can reserve tickets over the phone (the 24-hour SRT hotline is 1690 and they have English-speaking operators), because tickets must be paid for at one of many train stations in Bangkok by 22:00 the next day. The official website is not easy to use: tourists report problems with incomplete translations on the English version of the website, as well as with registering an account and setting up payment methods. If you search for Bangkok no trains will be found. This is because from 19/01/23, SRT trains leave Bangkok from the new Krung Thep Aphiwat Central Terminal, so use this name to search for Bangkok departures or arrivals. Some tourists prefer to buy from agents like 12go, that have easy-to-use websites and English-speaking customer service. All agents charge extra fees for their services.
A high speed railway is under construction between Bangkok and Vientiane, Laos, with a plan to continue it southward into Malaysia and Singapore.
1 Hua Lamphong Station. An old stately terminal, this is no longer the main railway station in Bangkok. It's central, near Yaowarat and has its own metro station. It's a charming old building of 1916, yet surprisingly efficient and convenient. On timetables, it's usually called "Bangkok" in English or "Krung Thep" in Thai. Tickets for commuter trains leaving the same or the next day can be bought at the counters under the big screens. The only intercity train that terminates here is the Eastern Line from Ban Klong Luk on the Cambodian border.
2 Krung Thep Aphiwat Central Terminal. Unofficially Bang Sue Grand Station, this is now the terminus for most inter-city trains. The station is directly connected to the MRT Blue line station of the same name, and is connected by Bus 52 to Mo Chit, for the long-distance bus terminal. It is the largest railway station in Southeast Asia, with 26 platforms; some 600 metres long.
3 Thonburi Train Station (ธนบุรี formerly Bangkok Noi Station) (west of the river in Thonburi). The terminus for twice-daily trains to Kanchanaburi (via Nakhon Pathom). Just to keep things confusing, the previous Thonburi Train Station right next to the river (accessible by the Chao Phraya Express Boat pier Railway Station) is now mothballed and turned into a museum, but it's only 800 m away from the new station. The weekend-only second class air conditioned tourist trains to Kanchanaburi and Nam Tok depart from Hualamphong Train Station.
4 Wongwian Yai Train Station (วงเวียนใหญ่) (About 800 m from the Skytrain station of the same name. To get there, take a metered taxi or walk (using a map)). Serves the rustic Mae Klong commuter line to the fishing village of Maha Chai. Trains run roughly hourly and the trip takes about one hour. The ride is of little interest if you want to get there fast, but is an experience for rail fans and an attraction in itself, with a nice view on the countryside's orchards, vegetable plantations and coconut groves. Maha Chai is a nice seafood destination, and if you feel like it, you can cross the Tha Chin river by ferry and continue by rail to Samut Songkhram. Tickets for trains departing from other stations are not sold at Wongwian Yai Train Station.