I've tried some of the Thai apps recommended by locals. Too many bugs. Grab is the only one that is stable.
BTS App is improving but still a long way to go. As long as Line Pay and Rabbit have a connection it will probably be a long time before they bother to make an online system for tourists.
Download a BTS/MRT map instead and use a Rabbit card. There is no way I am trusting Line with my credit card info. Supposedly one can now top up the Rabbit card on the machines at the stations as well.