I've received some questions over email about getting around by bike and train. One question that keeps coming up is that of whether a folding bike is necessary. This year it so happens that I've done bike+train trips a lot, with both full sized bikes and my (Brompton) folding bike. Here are some notes about the differences as I see them:
Peak vs Off-Peak Timing
In many cities, there are restrictions on the times of day that full size bikes are allowed on the subway and/or commuter rail. These restrictions coincide with popular commute times. For example, in Boston if I want to take a standard size bike on the subway in the morning, I need to do it before 7am. In the evening, I cannot do it until after 7pm. The commuter rail does not have such restrictions, but nonetheless it gets so crowded during peak hours that riding with a big bike is a nightmare. So I would say a folding bike is beneficial, and possibly even the only option, if you need to take public transit at peak commuting times. If your commute is entirely off peak, it may not matter.
Train and Platform Access
In greater Boston, many commuter rail stations have no elevator or ramp access down to the platform, only a long, steep, narrow staircase that gets slippery in wet weather. I believe that all subway stations do have elevator access, but it can get complicated with as many as 3 elevator changes to reach your platform. Dragging a full sized bike up and down long flights of stairs, or having to make several elevator trips with it, is not for everyone. Similarly, in some stations boarding the train itself requires climbing a precarious, steep set of stairs, whereas at other stations the train is flush with the platform and you simply step on. If the stations at which you board and disembark have inconvenient platform or train access, a folding bike could really be a godsent. If access is easy, it may not matter.
Handling Your Folder
All folding bikes are different. Make sure that yours folds sufficiently small to comply with the local transportation authority's definition of "folding bike" (some do not). Make sure also that you can comfortably lift and carry it for stretches at a time. Finally, make sure that you are capable of quickly folding and unfolding it yourself. If you cannot do these things, you may find multi-modal commuting with a folding bike more trouble than it's worth compared to a full sized bike.
In addition to taking your bike on the train, you will presumably also have to ride it to and from the station. Those who ask me about folders often express concern about this, especially if their trip is more than a couple of miles and involves hills. "Can you really ride all that way on a folding bike?" Well, I can on mine, yes. It is a distance-capable bike with a pleasant ride quality and low gears. But not all folding bikes are designed to be ridden for more than a couple of miles at a time. If the cycling portion of your commute is challenging, make sure to get a folder that is appropriate for that. Otherwise you may be better off with the full sized bike you know and love.
These are the points that come to mind from my experience so far. If you have questions that I did not address here, please ask in the comments. And if you use a folder for multi-modal commutes, please feel free to share your own experiences.