I'm not aware of anywhere that states rail customers are requested to sign in to take photos while changing trains during their journey.
I'm not sure if we agree but just aren't making ourselves clear, or if we do actually disagree here. To be very clear, if you're changing trains and you just want to hang around and take photos while you're doing so, I don't see any reason why you should need to sign in.
But if you're waiting around a station for a considerable period of time (I would say over an hour), standing in public areas but where there is nobody else around (including staff), such as on platform ends, then I would consider that you're there at least in part as a rail enthusiast. In this case, the
enthusiast guidelines would apply (at large stations presumably the
Network Rail ones which say largely the same thing). These aren't requirements or hard and fast rules, but I don't see what's wrong with following them where it seems reasonable to do so, in order to satisfy staff that you're there as an enthusiast and not as someone contemplating suicide or someone there for some untoward purpose, and that in the event of an evacuation, given they might not notice you in an out-of-the-way corner of the station, they know to look for you should something happen and you not make it out. I'm sure it puts staff at ease to know that the person standing ominously by the "no trespassing" sign isn't about to walk on the railway and get themselves hit by a train, but is just taking down unit numbers (or whatever).
I'm not saying that we should go out of our way to follow silly requests. Not at all. If a member of staff acts like a total arse towards me I'll likely wait until they're looking elsewhere and then carry on with whatever it was they objected to. But especially where I get politely requested to sign in when I'm there for long stretches of time, I'm inclined to follow that request, as I really don't feel it's that unreasonable.