Welding on a railway tank car, curious how/theoretical

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Still newbie enough to look and see when various welded items are within range.Today at work was clearing snow and the main CN rail line passes very close to the building, as the train slowed I could get a good close look at some tank cars, round general shape with a slight bell (belly?) on the end "caps", these particular ones carrying liquified anhydrous ammonia (sp?).There looked to be sort of a cradle shape between the body of the tank and the wheels at each end but it really appeared as if this cradle was not welded directly to the tank itself but rather it was resting on the cradle in an area of the tank where the walls were slightly sunken/depressed and then, once the tank was in place, flat plates with a curve matching the shape of the tank were welded to the tank walls locking it to the "T" shape of the cradle material.Anyone here build, repair or maintain large tanks, particularly rail car tanks?Curious as to the whys/wherefores of this method of attachment.Thanks, Russ
Reply:I'm not really familiar with them. But I'm willing it's to allow room to expand/retract, or simply to just flex and not crack.IW Local 580 NY, NY
Reply:I imagine it just increases surface area to hold the tankEd Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:I believe those tanks are double walled, with the inner tank being the structural part, and the outer existing for insulation and protection.Since the inner tank is what bears the weight of the liquid, I think you're seeing the cradle reaching into the interstitial space to attach to the inner tank.