I know you have counter argued me on this several times but at the same time its also getting hard to sympathies with lines when you hear 'Funding appeal is required to save line now' but then you see little to no ads and the lines rely on word of mouth or articles in papers its fair for enthusiasts to call for more promotion and i think some lines should try and ask for funding appeals so that they can afford these ad spaces specifically as many enthusiasts volunteer/armchair would agree that something does need to be done to invite a new audience in to try out a heritage railway.(1) A short campaign for the Wareham service would cost more than SRCs entire advertising budget for a year.
(2) Many heritage railways believe in the "Field of Dreams" approach.
(3) Many heritage railways see more value in putting the money towards restoration (loco, coach, wagon etc) than in marketing and thus revenue generation.
For a bit of a mini case study down here in Kent, i have seen ads for many non profit and charity tourist attractions such as Zoos, castles and one of Kent's biggest attractions, Chatham dockyard. They take advantage of ad space whenever possible be it bus billboards, Radio ad breaks, and railway posters on the national network so that people can appreciate that they are something to see in the area. Heritage railways for some are a bit of a 'buffet tourist attraction' where many wouldn't typically consider it as their first day out idea but if they do go and see it they might actually enjoy visiting these attractions in the same way we all do on here.
I agree with Brush 4, heritage railways have had some fun ad campaigns and some of the posters from the pioneering decades (60s-80s) are collectables in their own right now as the original big 4/Br ones are.