I have seen Somerset and Dorset Railway signs, light weight and in some cases a little thought would reveal that they are a joke. However, having worked on the Somerset and Dorset in 1965 I can tell you that the original signs are heavyweight in every respect, depth of lettering etc. all in significant releaf and painted usually red and white. Nothing tatty about these things except maybe the renewed painting.
Another sign that I saw advertised, and even when I challenged the seller he would not agree, referred to an amalgamated railway company but the date on this supposed genuine sign pre-dated the amalgamation ! A forgery too far back, you might say. Also if it looks to good to be old, it probably is not old... Another problem can be lamps, which are often described as "railway" presumably in order to inflate the price. Ships lamps, carriage lamps ( Horseless or horse drawn), and often road lamps all masquerade as railway lamps. Sometimes they can be genuine rather than fakes, and may be worth collecting, and often are, but are misdescribed. I have just bought a Fire Bucket. It is genuine I am sure, but is now only fit for a sprinkler system ! That was not what I intended as I wanted it for just what it was advertised as - a Fire Bucket not a sieve.