This is a video from a guy who regularly takes the MRT in Singapore with a foldable bike. He has given guidelines to help people who are interesting in commuting with a folding bike on the MRT or similar mass transport system.
Based on your experience level, you can either fold your bike completely and put it in a bag, carry the bike fully collapsed but not in a bag or, as the video shows, you are roll your bike along in a semi-collapse form like a trolley. Here’s a complete blog on how to convert your bike into the 3 modes (bike, trolly, carry).
For a complete breakdown of the 3 different methods of traveling with a folding bike on the MRT, check out this blog.