The FRA has a subsection on placement of placarded cars within the train. You need "buffer" cars between the engine and occupied caboose, and a bunch of other rules on what cars can be next to those placarded cars. That being said, I agree that another tank car of water would be a pretty bad idea, not to mention add extra weight to the train that most railroads wouldn't want to bother with.
In the real world, these "buffer" cars, when necessary, are various empty cars. Sometimes flats, but most of the time I see them being old beat up low-wall gondolas (the good ol' "Chippies" cars in Trainz) that don't have much use in today's railroading world.