I tend to agree that having one bicycle, the Swift is very nice. If I had a hybrid mountain bicycle the OP was referring to, the Swift *could* be jettisoned. And I could get something smaller.

But I don't think I could deal with the Bromptons. I need gears, and the odd gear shifters on the Brompton I tried threw me a bit. Also the brakes, the rack-as-a-kickstand feature are not for me. I like that Brompton seems to have good customer service for parts though. Unlike (cough) Dahon (cough). Lastly, compared to the Swift, the limitations with compatible accessories would annoy me. e.g. can't use this accessory, because it interferes with the fold etc etc.

If I really wanted a fold-to-small bicycle for a train, I would consider a Brompton, but also a Birdy, maybe.