It looks like he was in the Northamptonshire Regiment and not the Essex regiment, but you make an interesting point. It seems as though the Northamptonshire Regiment was also drawn away from the battle of Modder River (as an example) to guard the railway tracks. This would make sense in the context of Pepy's service.
Thank you for your knowledge and everyone else who has contributed so far. Now to find a photograph!
That is an especially nice group you have shown.
Regarding the title "Railway Staff Officer", some time ago I did a topic on a NSW Special Service Officer, Lt. Brace; who amongst other things, served as a Railway Staff Officer. Intrigued as to what that tittle actually meant, I finally fell back on the "Detailed History of the Railways in the South African War, 1899-1902" (two volumes). And there it was explained.
So I don't misread the entry or otherwise muck it up, here it is -
I don't think your man was guarding the baggage/railway line behind the action at Modder River. The qualifications for that clasp were rather wide and the Northamptons received the clasp under the heading of "All troops under Lt.Gen Methuen's command who were North of Honey Nest Kloof and South of the Magersfontein ridge, 28.11.1899".
Nevertheless, Lt. Pepys' duties during the campaign were - frankly - vital to the war effort.
This topic has been especially useful as I had completely forgotten it contained a listing of Jameson Raid members.
Thanks, Andrew and David.