The railway through Stow (opened 1848) ran between Edinburgh and Galashiels and was operated by the North British Railway Company. By the date of the accident the line had been extended southwards to link up with other railways at Carlisle.
This accident doesn't seem to have been reported on by the Board of Trade, whose inspectors investigated major railway accidents, but it might be mentioned in the NB Railway Co's minute books - now held at the Scottish Record Office, Edinburgh (West Register House). If it is mentioned there, the details may well be very brief, and confined to some such remark as "£5 awarded for funeral expenses." If the NBR's staff records survive (I don't know if they do, but if so they'll be in the SRO too) there may be some basic details, but probably nothing about the guard's family.