There is no refund for boarding the wrong train. You are required to purchase a new ticket i.e. KLIA Ekspres Single Trip (one-way) ticket for your journey on KLIA Ekspres train from the airport to KL Sentral Station.
To avoid such incidents in the future, please ensure that you have bought the correct ticket and boarded the correct train service. Please do no hesitate to ask for assistance from our Ticketing Officers should you require further clarification.