It looks to be a typical tool box at first glance... I wonder if flags or marker lamps / classification lamps were kept in here.
When I zoomed in to the picture, it seems the top and or the back of the box opens. It is too grainy to tell for sure, but I doubt it would be real easy to open the top of the box with a light mounted to the lid!!! I would assume that what ever it is, normal railroad related materials were kept in here. In addition to possibly flags or lamps, things like spare air hoses or tools could be stored here too.
Casey