How long did "Paytrain" remain as a brand, incidentally?
I'd say at least from the late 1960s until the late 1980s, but the concept remains in situ to this day albeit on a diminishing number of routes as ticket vending machines and ticketing websites and apps have reduced on-board ticket sales in recent years.
In the 1980s I remember "Pay Trains" (space between the two words) being referred to in on-train signage directing passengers on how to buy their tickets ("On Pay Trains, ....") Specifically, I remember the Reading-Gatwick/Tonbridge 119s having this signage (even though most stations were staffed?) as well as the 108s used on the Mid-Cheshire route, in both cases in the early-to-mid 80s.
As I recall the full wording of those signs was something like: "On Pay Trains please state your destination to the guard and tender the exact fare if possible. Check that your ticket shows the correct destination and fare paid, retaining it for examination or collection.
"Passengers are reminded that a valid ticket is required for each journey made."
A lot of first generation DMUs had those signs in the passenger saloons in the 1980s, even if they were also sometimes used on non-Pay Train services. I also seem to recall that some Pacers and Class 150 Sprinters had them when new.