Hello again Ruth,
First of all I am sorry to say that there is no record of Charles George Bunn having been a member of the Railway Pioneer Regiment.
I checked the Nominal Roll list first and then checked the enlistment forms to double check..
Is 'your' Charles the same Charles as Charles George Bunn born OND 1868 - Depwade - Vol: 4d - Page: 221 ?
If it is Ok with you I will have a look at a few other things when I am up at TNA this week, you never know we might be able to find him.