Southbound train near Nigg Bay just to the south of Aberdeen. The North Sea is in the background. The engine usually worked as far as Perth before being turned to return to Aberdeen. A Caley horse box leads the train. The loco number starts with a 9 and is one of the Cardean Class of the Caledonian Railway. There were only 5 of them built Nos 903-907. 903 was the only one named 'Cardean ' on the leading splasher, so it is not 903. 907 was written off completely in the 1915 Quintishill rail disaster, so if the photo is dated after that, before the grouping in 1922, that leaves you with 904, 5 or 6.
On further examination the second digit is definately a zero and it looks like the ast one is a 4 therefore it is almost certainly 904