Hi and apologies in advance if I have missed something glaringly obvious.

I have renamed my refuelling stop "Refuel L-CCCC" and yet despite my best efforts, no train maps to it when they run low on fuel, they simply die on the tracks.

Must all trains be assigned to a trainline before the automatic refuelling will function?

A bit lost as to what exactly the problem is.

From the information page: "Checking Refuel in the trainline or train UI will add a refueling station to the end of the trains schedule, once that train needs refueling and remove it once it is done."

If you check the 'Trainlines UI' header on that same page, you can see there are 3 checkboxes for each line: Active, Marked and Refuel. For a train to refuel, you need to have a line active on it, and hit that refuel button.