BTS (at the mo I am i BKK-regular visitor) is funny: the oldest inner part is privately/commercial and by now super-heavy used, the newer outer strecthes are owned by the BMA=Bkk Metrop. Area/agglo and have differrent and much lower fares-cross the borderline and you are charged both fares. The latest openings are mostly free for a month or 2 (currently north from Mochit now as far as Kasert Univ.-and soon to be extended a few more stops north-back side of old airport Don Muang. On BMA-owned (but BTS served-trains simply run though) sectors the flat fare is 15 bt otherwise.(about 0,45 Eur or o.40GBP). That may even be lower as the fare on AC-buses that run underneath them.
But the longest new stretches are on the MRT/BEM-the state-owned underground, started as metro and not much used, but this has changed. New extensions are mostly also up in the air and just a week or 2 ago the loop has been closed, the main line is now like a P with the long pole east-west and a change point (Ta Pra) on 2 levels where the loop meets the stick. The new extensions only have trains from 10-16 but are also free.
BKK has 2 airports and the major INTernational one is Suvannaphumi (mostly nicknamed Swampy-its in a large former swamp with snakes etc.) and it does have a ARL=Airpt RailLink since some 5 yrs, run as a special divison of the SRT=state railways. The old airport is Don Muang (DMK in IATA) and has a train statioon just across the rd since as long as its there-but the service is not over-abundant and INto town pretty unreliable due to giant delays. But here the state SRT is also building like the ARL a kind of skytrain above the rails (called red line) and this is due to open late this/early next yr. with also the giant new main trainstation (built for the new to come hi-speedo trains to China). In due course to be linked again above the rails with the ARL to other airport.
The main complaint is that the then 3 systems (and then the buses and the boats) all have their very own fare systems and cards and refuse to accept the other ones. There have been already 2 trials to start with a new universal for all card (Rabbit card now-chip with stored value) but its not universal at all. This is fairly tipical for how things run anywhere here in TH-noone will trust another one, not even state or city. Compare that with how Malaysia got it reorganised in KUL, let alone the Chinese cities.
Look-as for about any new or existing metro system on urbanrail.net for more info and on the skyscrapercity forum for discussions and plans and constructions. Mr. Wisaruth is a former SRT-employer busy with informing all of us with the developments.