This includes (maybe not exclusively) Steam Locomotives, Boilers and Hobbyist's Steamengines

Actually if you put in ic2 cells, you get the same result. What a ..? And Forestry cells do so.

fair enough

Many fluid containers are intended to be single use. Railcraft simply follows the standard that if the bottle wants to return something after being emptied, it needs to implement Item.getContainerStack(), just like buckets do.