The answer
Take the SRT Dark Red Line — 33–42 THB, about 20 minutes to Krung Thep Aphiwat (Bang Sue), where you step onto the MRT Blue Line. Follow "SRT / Red Line" signs and take the walkway from T1 or T2. Ignore every taxi tout between you and it.
Trains run roughly 05:00/05:30–midnight. Buy the token at the machine — coins and notes accepted.
Staying near Khao San or Pratunam? The A1–A4 airport buses (30–50 THB, cash on board) go direct — A4 to Khao San, A3 to Pratunam/Lumphini. From T1 Level 1, Gate 6.
⚠️ The traps (walk past these)
The flat-fare taxi offer
Touts inside and just outside arrivals offer "fixed price, no traffic worry" rides at 800–1,200 THB — triple the meter. A metered taxi to Sukhumvit is 300–450 THB even with surcharge and tolls.
Do this: walk to the official taxi rank, take a kiosk ticket, insist on the meter.
The "train is closed" line
Drivers near the walkway tell travelers the Red Line is closed / slow / not running today. It runs ~05:00–midnight, roughly every 10–20 min, and it's 33–42 THB.
Do this: check the SRT timetable yourself on the platform screens. The walkway is inside the terminal — you never need a tout's help to find it.
The wrong-airport shuttle
The free DMK ↔ BKK shuttle is only for passengers with an onward ticket from Suvarnabhumi — staff check. Backpackers queue 30 minutes and get turned away.
Do this: no onward ticket? Limo Bus Express (150 THB) or a Grab between airports instead.
Bus fare exact-change scramble
The A1–A4 buses are cash on board. Conductors can usually change notes, but at rush hour with a 1,000 THB note you'll hold up the bus and get waved off.
Do this: break a note at a 7-Eleven in arrivals before heading down to Gate 6.