I've been doing this for nearly five years now, so I've gradually built up a system that works for me. Usually I'll use one of two devices:

* Asus Chromebook Flip (C100PA)

* iPad Pro 10.5 with keyboard cover

When I'm developing, I use ssh or mosh to access my workstation in the office and then tmux+vim. Mosh works pretty great with poor connectivity. I've pretty much mentally mapped the signal quality along my route, so I know when to look out the window for a couple of minutes to wait for signal to come back...

When I'm not developing, I'm usually using Gmail+Google Docs/Sheets/etc to get stuff done. They all work relatively well offline/online.

For connectivity, I use a Raspberry Pi Zero with a 4G data stick as a WiFi hotspot (the on-train WiFi is terrible, and tethering to my phone kills the phone battery way too quickly). That setup can run for 6-8 hours on a little Anker USB battery pack.