This happens for me automatically: I quite often run out of writing time before I get to the end of the scene I’m writing. I can’t, obviously, sit on the train and finish it off, or I’d end up being taken straight home again, so I have to stop. Mid-sentence, sometimes.
Also, sometimes I run out of steam at or around Finsbury Park, so I shut the notebook and look to see whether there’s an update from Lady B-, the Comfortable Courtesan as was.
On the train home, I often find that the morning’s part, which I’d thought would only take a paragraph or so to wrap up, wants to run on and on into the next scene. Or a different part of the book entirely. I’m not fussy.