To be fair, people do this ALL THE TIME on the NYMR and Ffestiniog, and the driver's don't mind, in fact - they regularly toot photographers. I don't know if a preserved 'heritage' railway has different by-laws, but I'm sure someone who knows better will confirm.
I seem to remember a similar discussion not so long ago about the legality of photgraphing trains from the pedestrian swing gate styles on some level crossings. I think, if I remember the verdict correctly, there is nothing technically wrong with being inside a level crossing when a train is coming providing you do not attempt to cross the line whilst the sirens/barriers are on/down because the exact legal writ is 'do not cross the line' rather than 'do not be in the pedestrian style'. Someone will no doubt correct me if I'm mis-remembering etc...