'Tank iron' definitions:

Definition of 'tank iron'

noun

Plate iron that is thinner than boilerplate but thicker than sheet iron or stovepipe iron

From: GCIDE

Tank \Tank\ (t[a^][ng]k), n. [Pg. tanque, L. stangum a pool; or perhaps of East Indian origin. Cf. Stank, n.]
1. A large basin or cistern; an artificial receptacle for liquids. [1913 Webster]
2. A pond, pool, or small lake, natural or artificial. [Webster 1913 Suppl.]
We stood in the afterglow on the bank of the tank and saw the ducks come home. --F. Remington.
The tanks are full and the grass is high. --Lawson. [Webster 1913 Suppl.]
3. (Mil.) a heavily armored combat vehicle which moves on caterpillar treads, rather than wheels. It typically carries a cannon and a heavy machine, and sometimes other weapons. It is the main distinguishing weapon of an armored division. [PJC]
4. a jail cell for temporarily holding prisoners, as in a police station. [PJC]
Tank engine, a locomotive which carries the water and fuel it requires, thus dispensing with a tender.
Tank iron, plate iron thinner than boiler plate, and thicker than sheet iron or stovepipe iron.
Tank worm (Zool.), a small nematoid worm found in the water tanks of India, supposed by some to be the young of the Guinea worm. [1913 Webster]