The site "UK Railway war memorials" states that the IWM records around 400 memorials 35 of which were recorded by Railway companies prior to the 1923 Grouping. Significantly several not recorded by the IWM have been located.
However, some company memorials particularly those in Wales incorporated into the GWR and the Didcot, Newbury and Southampton Railway have not been located. So, something to look out for I would think.
On the subject of legibility of the memorials the limestone memorial to staff of the NER located in York has become very difficult to read in parts, but I understand there is a full transcription located at the NER, also in York
