Don't quote me on this but I think they came with the railcars so either they were removed from other railcars and put in them when the ARHS got them done up to run a few years back or else they were in there from before and just left there. I could be wrong on this though, anyone know for sure!
The photo boards where obtained from former STA buses many years ago, prior to the NRM actually getting the RedHens. We kept them with the intention of putting them in when we eventually got a rail car set. The buses, which had been brought by someone that had intended doing them up, had the same adverts as the rail cars. When the bus owner decided to get rid of the buses we found he had kept the adverts in a corner of hi depot shed, so we grabbed them while we could.