Here is Vicsig's description of the Sandringham line: "The line operates as a suburban electric service in both directions, with all services stopping all stations. A limited number of empty car movements operate, as well as transfers of units being modified at Brighton Beach." So modification works did occur at Brighton Beach sometime in the past, and I assume the fences were put in during that time. I haven't had a look at the fences personally so I can't really comment on their layout.Ah that rings a bell it would have probably been for VICERS (vigilance controller I think, I was too young to remember when it was going) many years ago, I saw it on vicsig but I wasn't sure if it was up to date. I can see why the platform would have been included in the fencing because they would have needed it for the works and maybe they decided after the works had finished to keep the existing fencing, considering that the back platform (to them) isn't necessary and is more cost effective to close it. IMO they should either convert it to a stabling road or use it to terminate trains when needed.
Platform 1 was fenced off while work was being done on trains for the VICERS project. As far I am aware the platform is too short for 6 car trains so there was probably no point in removing the fence & reinstating it once the project was completed. The two trains that stable at Brighton Beach overnight run empty to Middle Brighton & start in service from there.Would it be worth extending Platform 1 to fit a six carriage train so it can be reinstated for short services? If not, (as I think the station is heritage listed, creating more difficulties), surely passengers can be advised at stations up the line that for Brighton Beach to just board the relevant carriages at each station, don't think this is an issue for just one station, especially one that is not particularly busy.
You do have a valid point of availing it as an emergency terminating location, but the possibility of needing it would just be the same as any other parts of the network, and it doesn't justify for a crossover at every station. With the peak frequency of 8 minutes a two-tier service won't be needed in the near future. Express services also aren't really useful without overtaking provisions. Say you had an up Sandringham express departing BBH 2 minutes ahead of a BBH short start SAS, it would only gain 4 minutes before catching up with the previous SAS, envisaging that 2 minute is the shortest possible headway. 4 minutes doesn't really mean much for passengers.I must say its impressive that the frequency of 8 minutes can be maintained with only one platform at sandringham! I didn't advocate for express services on the sandringham line it seems pointless (I remember hearing of BBH plat 1 last regular use in the 70's when there was a swing door service, but these days all trains run through). To me it seems a waste that the platform sits there idle but as you say there is very little need for it (totally agree with you there would be zero point building crossovers at every station, its just the infrastructure exists at BBH that's all). Also what's more wasteful is the tracks for that road are only 10 years old (relaid 2010) and have never been used once since that date (to my knowledge). The last train to "officially" use it (not counting VICERS which saw the closure of it) would likely be when they removed the Mtrain livery or (from what i hear from old forums) when there was a disruption at Hampton many years ago.