There's one line running.

It goes fom Hualampong train station in the city centre along Rama 4 road, (passes the north end of Silom Road / Lumini Park) then heads north past Queen Sirikit Exhibition centre and crosses Sukmuvit Rd- connecting with Asoke skytrain stop.

It continues up Rachdapisek road - look for good deals on business class hotels in this area, most tourist visitors will have previously igniored them as there was no easy access to city centre - which there is ow thanks to the subway. It then loops around Lad Prao and ends up at Morchit where it's a 5 minute walk to Chatuchak weekend market and you also connect to the skytrain.

Other routes are underway but none of these will be open for a couple of years

It certainly opens up the range of hotels which I would not have considered before. Lets face it, you had to be near a skytrain stop or getting from one place to another could be a nightmare.