Mass transit and bicycles are possible using folding bikes. Transit systems often exclude full size bicycles from bus and rail. The exception being the folding bike. What's holding back cyclists from using folding bikes with transit is the perception the small wheels makes them look silly.
This has been my experience using bus and trains
Trains - I've never had any problems using folding bikes with rail. During ruch hour, folding bikes are the ONLY cycles that can ride on every train providing they are covered.
Bus - This is a tricky situation. The folder would have to have a 16' inch wheel in general. If the bus is fairly large or has a storge compartment on board, a 20' inch wheel folder might do just as well. Keep in mind that bus drivers DO NOT like folders at all and if they suspect what's in the package is a bicycle, you will not be able to board. Always have the bike folded and bagged before you board the cabin of a bus. Unfortunately, once you bag a folding bike, the package becomes larger and will inconvience other passengers. During rush hour, you may have to stand with the folder off to a side if possible. If the bus is so small with no storage compartment and the only place to put it is on the seat next to you, a folding bike is NOT an option. A folding bike SHOULD NOT go on the overhead rack.
All buses are different and situations can vary. I'd say this. If you notice other people on many occasions boarding with large packages or suitcases during rush hour, then a 16' inch wheel folding bike is certainly an option. Many buses have certain sweet spots where you might be able to place a folder without affending other people. It certainly helps to board the bus at the begining of its route before the cabin gets full.