KLIA2 has its own Ekspres station on the same line to KL Sentral — RM55 adult one-way, about 33 minutes from terminal 2. Buy on the app for roughly RM49.50 (10% discount). From KL Sentral, monorail to Bukit Bintang is RM2–3 with Touch 'n Go, or Grab RM15–25 off-peak. KLIA and KLIA2 are 2 km apart; a free shuttle runs between main terminals if you accidentally enter the wrong one, but that wastes 15 minutes. AirAsia arrivals: immigration is at KLIA2 — walk straight to the train signs after baggage, don't exit to the curb unless you're taking Grab.
Same train, different station building. Follow signs to KLIA Ekspres inside KLIA2 — don't get on the free purple shuttle between terminals unless you bought the wrong ticket.
We grabbed from KLIA2 at 11pm — RM78 to KLCC, no surge. Fine with kids asleep, train would have meant two changes.