I'm going through this thought process right now. My potential commute would involve a bus (with very limited rack space) rather than a train though. The riding distances would be nearly identical.

If you can *always* get your conventional bike on the train it seems like a no-brainer to me. If full sized bikes are restricted during certain hours or your bike parking situation at work sucks those would be good reasons to keep using the folder.

If the job thing works out I imagine I'll build up a Xootr Swift frameset singlespeed with drop bars. I love the fold of Bromptons but unless I have to fold my bike for every single trip it doesn't seem worth the financial hit and less than ideal riding position.

I imagine in my situation availability of bus racks could fluctuate seasonally too (empty in January, full in July). We'll see.