Talad Rot Fai (='Train Market') is a popular night market on Srinakarin Road behind Seacon Square. The market opens at 5 pm and goes on till after midnight. It is an open-air bazaar featuring rows upon rows of hundreds of tent shops selling all kinds of wares such as clothes, bags, shoes, accessories, knickknacks, souvenirs, antiques and also food.
The bazaar is located in the south-east of the city center, as highlighted red in the map of Bangkok below.
Contrary to the name, there is no train passing by the market, neither is it connected to the city's mass transit lines (BTS/MRT) yet. So it is best to get there by taxi. The market is right behind Seacon Square - one of the largest shopping centers in Bangkok. Seacon Square and the night market would together make up an ideal day trip for your shopping goals.
Note that there is another train market also called Talad Rot Fai (Ratchada) located on Ratchadapisek Road, nearer the city center and accessible by MRT train. To avoid confusion, when getting to these markets by taxi, make sure to tell the driver the road name as well - "Talad Rot Fai Srinakarin" or "Talad Rot Fai Ratchada".