There had to be at least one for the guard, plus heating and lighting.
1. You had 4-wheeled guards / parcel vans with no genny - they were first. These were the ones initially known semi-officially as “hooded vans”, later less officially as “tin vans”. In official parlance they were abbreviated to “LV” (= “Luggage Van”).
2. Then the same but with generators, a little luggage space, and of course, the guard. These were known as “HV”, or “heating van” to some, “GSV” (generating steam van) to others, and “hot water bottles” to enthusiasts.
Both of these were used on most trains from the late 50s, but all trains in the 60s, bar the odd rare exception like the solitary and unique Loughrea coach.
In grey 121 days, or black’n’tan days, tin vans of both types above are quite simply essential.
Then,
3. The mail vans. There were a small number built for use on mail trains. These had neither heating boiler nor brake, but at least one was subsequently rebuilt as a brake vehicle, despite retaining its TPO appearance. I think that’s the one now at Downpatrick, following its later career in the weedspray.
4. The later 6-wheeled vans (in 1964/5) were heating / guard only - none were built as LVs only. These were only ever in black’n’tan.
1, 2, and 3 started silver, but most were eventually painted green, especially the “hot water bottles”. Some made it to black’n’tan days still in extremely filthy “silver”.