Hard to tell: it looks to have a filler on the top, so may well be a sandbox -it is certainly big enough, but they are often positioned so the sand is fed just in front/behind the driving wheels. It looks too big to be an oil reservoir, and the small circular lid suggests it probably isn't a toolbox.
The previous item is a traversing jack: they have a vertical thread for lifting the locomotive, and a second one to move it sideways, back onto the track. As others note, they often appear where track is poor.