Resurrection of an older thread! :)
I bought a Brompton in 2009 to use in a bimodal commute to work. The Brompton took care of "the last mile" of the commute, bookending a 30-minute train ride. While Caltrain did (still does) allow bicycles onboard, in those days, there was only one train car that was set up to accept bikes. Those bike cars were often oversubscribed, leading to cyclists getting "bumped" (i.e., unable to board). A Brompton (or any folding bike) allowed one to bypass that restriction because they were not considered bikes but luggage (like backpacks) when folded.
My Brompton remained in service when Caltrain added a second bike car to every train to alleviate "bumping", because it was fine for my "last mile" that was really 2 miles; much longer, and I'd had preferred a "proper" bike because I find my Brompton a bit flexy, takes a tad more effort to keep speed, and the gears on the 3-speed IGH are spread too far apart (33% jumps if I remember correctly, corresponding to almost 3-gear shifts on a more conventional "road" cassette). Nowadays, with the post-pandemic train ridership being dismal, and my employer offering a generous office-remote ratio, my Brompton sits mostly dormant.
Note: if you're considering a Brompton, always test ride with a front load. Bromptons have low trail front geometry intended to carry front loads, and can get a bit squirrelly if there's no front load. You can get used to this squirrelliness, but it often catches riders who are used to more conventional mid or high trail geometry (of most other bikes) off guard. I often question why Brompton dealers send test riders out without a front load, because I'm sure they lost some potential business from this quirk.