Shields or plates attached on the ends of the cars are to protect from couplers piercing the tanks?
Bingo!
Tank cars also have shelf couplers, which help keep the cars coupled as well.
Not all the tankcars have this is it depending on commodity?
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Maybe… not all tanks carry hazmat.
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Some tanks have the visible shields, some have thickened ends, or head shields underneath tank insulation, neither of which would be visually obvious.
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It’s a relatively newer thing and cars built 30-50 years ago and still in service don’t have them. Some of these older cars are being bumped out of hazmat service and/or retired due to the changing regulations.