This fake badge recently appeared on eBay. I've come across some documentary information on specific ARP teams attached to the various railway companies during the war but I have not seen any badges in period photos. There was a GWR ARP helmet that appeared sometime ago but the authenticity of it was questioned at the time as well. If you know more about ARP teams working for the railways please let me know, especially any photographic evidence.
Update: the seller of this badge and other items of militaria has a very dubious record in the authenticity of certain items. The majority consensus from collectors is that the provenance of insignia relating to ARP teams on railways is highly dubious. Buyer beware...
News about interesting insignia, ARP related info and period photos that turn up.