The General Manager of the Sri Lanka Railways, Dammika Jayasundara stated that passengers can book railway tickets online from the 18th of February, 2022. (According to News 1st)
The online ticket booking facility will be provided via a website and a mobile app.
Link: https://seatreservation.railway.gov.lk/mtktwebslr/ (21)
Passengers will be able to reserve seats and make payments using Visa, Master, and mCash as you see in the images below.
Screenshots from Social media