Not got direct experience, but we have had at least one user on the forum successfully running Audacity on an EeePC (sorry, I don’t think they said which model).
How much horsepower you need depends on the project - the more tracks you have the more oomph you need.
I have a Pentium III 500MHz desktop machine that will run Audacity fine (up to about 10 simultaneous tracks)
Probably the most demanding part is the recording, which I guess you will not be doing on the train?
Playback and editing is less demanding.
The other consideration is that Audacity projects can get very large. Using the default 32bit format, each stereo track will use around 20MB per minute. Each time you process a file, the amount of data is increased due to the “Undo” files, so a 10 minute multi-track recording can quickly become a Gigabyte or more of data.