The line, which is two years behind schedule, will cut travel time between the heart of Bangkok and its suburbs to less than half an hour thanks to high-speed electric trains.
A three-month trial run kicked off at 10.30am on Monday, and the service is expected to start commercial operations from November.
The Red Line runs two routes – the 10-station 26km Bang Sue-Rangsit route which takes 25 minutes to cover and the three-station 15km Bang Sue-Taling Chan route which takes about 15 minutes.
Fares range from 12 to 42 baht.
The service starts at 6am and ends at 7.30pm on both routes.
Trains will run at an interval of 30 minutes during normal hours but every 15 minutes during rush hour (7am-9am and 5pm to 7.30pm).