I don't know why they don't use the old canvas water bags either...
I'll tell you that when I worked there, that water bag was absolutely essential....without it, you'd get yourself dehydrated and get plenty sick in a hurry. They held about a gallon or so, and we would drink the things dry two or three times during a trip on a hot day.
I suppose now there is worry about catching some kind of horrible disease or something, but 44 years ago we didn't much worry about it, and I don't recall anyone catching anything horrible or even hearing about anybody that did...All the engine crews used them, and they were issued to us when we hired on. Each man had his own and we always kept them filled or at least damp so they'd hold water during a trip.
They cooled the water nicely and it was always refreshing to grab the thing from where it hung just below the cab window. pop the cork and take a swig from it after you got back up on the seat from putting in a fire. We filled 'em at the water tanks when we took water if they needed it.