Honestly speaking, I really think that Bangkok train station is abit too complicated for me hahahah! Maybe because my sense of direction is really bad. Theres 4 main train line in BKK, ARL Airport Rail Link, MRT Metro Chaloem Ratchamongkhon Line, BTS Skytrain Silom Line and BTS Skytrain Sukhumvit Line.

Bangkok Train Station Map

Theres 3 type of train services in the airport. SA City Line, Makkasan Express Line and Phaya Thai Express Line.

Suvarnabhumi Airport Express-Makkasan (Makkasan Express Line):

• Travel directly to Makkasan/ Suvarnabhumi Airport without stopping (Non-stop service)
• 15min ride
• Round trip ticket: 150 baht (SGD 6.10)
• Single journey ticket: 90 baht (SGD 3.70)

Suvarnabhumi Airport Express - Phaya Thai (Phaya Thai Express Line):

• Travel directly to Phaya Thai/ Suvarnabhumi Airport without stopping (Non-stop service)
• 17min ride
• Round trip ticket: 150 baht
• Single journey ticket: 90 baht

SA City Line: 

• Service line between 6 stations
• Travel time from first to last: 30mins
• Max fare: 45 baht (SGD 1.90)

Its complicated because you can't change from one line to another within the train station. You need to tap out, walk for a few mins to reach the other line. For example, from Suvarnabhumi Airport Station (ARL) to Siam BTS station, you will have to first alight at Phaya Thai station (ARL), then tap out and walk over to Phaya Thai BTS line to travel to Siam BTS. But don't worry, there will be a signboard at every interchange to guide you the road from one station to another.

The maximum fare for each trip is 45Baht. From Suvarnabhumi Airport to Phaya Thai, it cost us 45baht per person. From Suvarnabhumi Airport to Makkansan station, it cost about 35 baht. From Siam BTS (Siam paragon/Siam Center) to Mo Chit BTS (Chatuchak weekend market), the fare is around 42 baht (SGD 1.70).

If you want to buy a one day pass which allow you to have unlimited ride for the day, do note that it only applies to their own line. Only the 2 green line, BTS Silom and BTS Sukhumvit are using the same day pass. For MRT (the blue line), you will need to buy a separate day pass. Of course, if you need to travel between different lines, its not worth to buy 2 different day pass. The day pass for BTS is 130baht (SGD 5.30), while the day pass for MRT is 120baht (SGD 4.90). There is also a 3 day pass for MRT which cost you 230 baht (SGD 9.30) per card. As for BTS, I think there isn't any 3 days pass. Correct me if I'm wrong. Thanks! :) Based on my experience, I normally wont spend more than 130baht a day using the single trip card.

One day fare pass MRT: http://www.bts.co.th/customer/en/01-ticketing-type-one-day-pass.aspx
One day fare pass BTS: http://www.bangkokmetro.co.th/ticket.aspx?Lang=En&Menu=65

BTS fare guide

MRT fare guide

Calculate fare (BTS): http://www.bts.co.th/customer/en/02-route-current_new.aspx
Calculate fare (MRT): http://www.bangkokmrt.com/mrt-fares.html

Some of the popular attraction near the BTS:

• Siam BTS: Siam Paragon, Siam Center, Siam Square
• National Stadium BTS: MBK Shopping Mall
• Chit Lom BTS: Central World, Big C, Platinum Fashion Mall
• Asok BTS: Terminal 21
• Mo Chit BTS: Chatuchak Weekend Market
• Saphan Taksin BTS: Asiatique Ferry Terminal/ Near Khao San

Hope this Bangkok train station guide helps! (▰˘◡˘▰)   
Please correct me if any information given is wrong! Thanks! ^^