stut. The airport rail link is standard guage, incompatable with SRT, and would require totally new infrastructure to reach Hua Lamphong.
Most inbound airline passengers who need to travel any distance take a domestic flight. The airport rail link is really only for those actually going to Bangkok itself.
The following is a little off topic, but may be of interest to anyone going to Bangkok.
There's a frequent dedicated bus from the airport to Hua Lamphong. Its Airport Express 4, (AE4), operates every 15-20 minutes so you will never have to wait long, leaves from Door 8, Level 1, ground floor, costs B150 flat fare, and sets down only en route. It terminates just a few metres from the station entrance and is ideal for those with heavy luggage.
I've only used Makkasan terminal once. It was about 2200, and deserted apart from a security lady who directed me to the ticket office, which was staffed by a booking clerk who looked as though he hadn't sold a ticket all shift! I had the entire train to the airport to myself.
For those who wish to travel to Hua Lamphong using the airport rail line, take it to Makkasan, then walk to Petchaburi underground station. Its only 200 metres or so, but involves crossing a busy road. From there you can either go take the MRT to either Hua Lamphong or Bang Sue. The latter is where all SRT Northern, North Eastern and Southern line services call.
For the really really desperate, there's another option, which is to take the airport rail link to Makkasan, then walk back 200 metres or so back along the main Makkasan Road to the junction with Sukhumvit 21 (Asoke) road. Go straight ahead at the junction, across the single line, and you may, if you really look, find Asoke SRT station. I say 'station', its officially a stopping place, unstaffed, and made up of hundreds of concrete slabs that look like sleepers but aren't. It has about 14 trains each eay per day, less at weekends, but the fare may put you off. It the current equivalent of 4p to Hua Lamphong, that's if the gripper can be bothered to do his job, and takes about 25 minutes. And, if that's not enough, although most trains are 3rd class only and hauled, you may be bowled with a Thai version of a 158 and charged extra for the privilege as they are second class only. You have been warned! First service to Hua Lamphong 0641, last one at 2112.