I'm also on a train a lot lately and I find that even with a a MiFi device, there are often times that my Internet connection is spotty. Knowing this, I have recently taken the time to update documentation for an upcoming project I expect to open source.
Writing in Markdown is pretty simple, but sometimes I like to check my work (i.e. to find bugs like missed closing code fences) before I commit the documentation to the repo. While there are plenty of online editors that can help with this, if I've not opened one before I've lost connectivity, I typically have to wait until I get closer to civilization. Also, I'm impatient, especially when I'm in the groove of writing.
Thanks to @jbt I can satisfy my need for immediate feedback when writing documentation, especially while on a train, in the sticks.