Thank information from www.squidoo.com/expat-tips-thailand
TS & MRT Rail Systems
Traveling in and around Bangkok is cheap and really easy. Bangkok has a great rail network that consists of the overhead BTS and underground MRT systems. Both the MRT and BTS have pre-paid cards or one-way ticketing systems. If you're going to be in Bangkok for a number of days, it's a great idea to pick up a pre-paid card available at any BTS or MRT station kiosks; one-way passes are also available at ticket machines located centrally at all stations.