I can think of a couple possibilities, none of which use the iBook directly.
There exist fold-out keyboards for palm-pilot and PocketPC devices, these are very small (large pocket sized), and can record to a small device. I don’t know of any for an iPod (the iPhone could do it, but the thought of typing large amounts of text on that keyboard makes me shudder). The fold-out keyboard/palmpilot combination has been around for most of a decade, so you might be able to find deals on eBay.
The other would be something like this: an AlphaSmart Neo. They’re slightly pricier ($219) , but you may be able to find the older models on eBay again. I’ve got one from a few years back: it’s larger than a palmtop but smaller than an opened laptop, extremely lightweight, and lasts basically forever on three AA batteries. They’re designed mostly for kids, but they’re used a lot by journalists and the like because they’re practically indestructable. It’s just a keyboard connected to a cheap 4-line or 8-line display and solid-state memory; basically just a “delayed keyboard.” When you get it someplace where you can get to a computer, you connect it with a standard USB cable, open the word processor of your choice, and hit a key: they keyboard will then “type” everything it has stored in that particular document (it can hold several “documents” at a time). It will also act as a USB keyboard for any computer that can handle one (Macs and modern PCs), which has saved me a couple times.
There’s also the option of plain old paper: if your handwriting is neat enough, a scanner + OCR can turn this into digital form later. My handwriting does not meet the “neat enough” criteria, so I don’t know how well this works (I’d guess, not well). But you may find that even transcribing by hand isn’t too onerous; how much text are you going to generate in a single train ride, anyway?