Most likely running into Platform 5, as it was in those days, to stand at North Signal Box to await its 'Down' train working. Doncaster South Box in photo with St.Jame's Bridge (and Church, the Plant Church) also in view. Balby Bridge, scene of a dreadful passenger train accident is beyond the bridge visible. The only other purpose would be if it was to take up the North End Standby Pilot duty.