What's the story behind this little lokey? That paint color has me thinking of Thomas the Tank Engine!
It looks to be almost like a fireless steam loco switcher. I read about them and they were pretty much how we use battery power today. They used a 0-6-0 configuration steamer and then there was a steam generator charging station that would charge steam into the boiler tank. Then the steamer would go about it's duty swithcing in the yard. I'm not sure how long it would go on one steam charge.
This would be a nice resto project.
Actually EMD trainman, the engines you are referring too have a much larger "boiler", which in this case would simply be a large air tank and would have no reason to have saddle tanks for water. It would appear the strange object on the top was simply an easy way to fill both saddles with water. And if you notice the bulge in the back, that is where they would store a small amount of coal.
If you would like to take a look at an engine that actually is powered by steam from an on-site steam generator, check this picture from the Pennsylvania Rail Road museum - http://www.nscalelimited.com/wp-content/uploads/2009/03/img_2337.jpg
Thomas the Tank Engine!!