i'm currently in the market for a bike to use around town and as a commuter as part of my public transit commute to and from san francisco on a daily basis, usually on bart. i've been looking around bikeforums.net for several days and figured i'd create an account as i'l likely be asking some questions in the upcoming weeks while i research my purchase.
currently i'm looking at a folder: Dahon, Bike Friday maybe, and i'm hoping to have a look at the Breezer i7. i've also got some interest in the strida.
Not sure about other local transit systems (from observation, many bus systems also support bikes). So I'd have to agree with iceratt: get a regular bike unless a folder is absolutely essential (or unless you just have a thing for folders, can't imagine why!).
Unfortunately, with BART, bike are not allowed on certain trains from certain stations during the heaviest commute hours in the morning and afternoon. For instance, bikes are barred from getting on BART at the El Cerrito Plaza station between 6:42am an 8:27am, if you are going to the City. However, if you are going south on the Fremont train there is no blanket restriction in the morning. However, there is the small print, "Bikes are permitted on this line when trains are not crowded." So that puts one at the mercy of someone else's definition of "crowded."
With CalTrain and the onset of "Baby Bullet" service, the bike commute picture has gotten a little muddier, and the prospect of a train having reached its bike max before you can get on has increased.
So, it looks like having a folder would increase the chances of getting on the train you want (in the case of CalTrain), or getting in a train at all (BART to the City from Albany).