The rotfai dtai din, as it is called in Thai, is a separate system from the rotfai faa, i.e. the BTS Skytrain. Because of this, travelers cannot transfer between the two using the same card. This may change some day.
The MRT starts at Hua Lamphong, the main train station near Chinatown, and heads southeast from there, making stops at Silom and Lumphini Park. After that it makes a 45-degree turn at the Klongtoei stop and heads north.
The Sukhumvit stop is right next to the Skytrain Asok stop, and Petchaburi, the next stop, is near RCA. The subway then heads through Din Daeng, a neighborhood with few attractions for visitors. Further on down the line is Chatuchak Park, though, home of the Weekend Market. The last stop is Bang Sue Railway Station.
Single journey passes are plastic tokens that cost between 16 and 41 baht, depending on how far you’re traveling. Tap the token on the right-hand side of the turnstile to enter, then send it into the slot to exit on your way out. Multiple trip passes are available, too — ask about them at the ticket window.