For crowded UK train commutes, a flapper style folder like the Dahon or DTs really don't work - too big, heavy and inconvenient.
I disagree, depends on the line, etc. I am increasingly surprised by the amount of people using cadenzas on busy trains - I wouldn't worry overly about secreting a 20" folder.
The tube is a different proposition - in the rush hour, the Brommie and Mezzo come into their own...