" Mary's father died in an accident at work on the railway in 1916 and it is noted he left a widow and three children."
Can I just ask if Mary was actually named including her middle name ?
There are 7 births 'Mary Campbell' (some with middle names) Hutchesontown 1912 - 1916
If Mary W did die young then it's quite possible a later child may have been named Mary in her memory, this did happen in families!
We may all have our thoughts wrong but until we know for sure the exact wording regarding 'Mary Walker' we're kind of at a standstill?
We are trying to help! 
Annie
Add...although I do find it odd, had 'Mary W' died, why her middle name or at least middle initial isn't recorded?
None of the children were named only that he left a widow and three children sorry