tank milk cars were produced by the General American Car Company in both 40' & 50' lengths and carried reporting marks of GACX among others. they looked very much like reefers built by GACC except for the lack of roof hatches. they were filled at creameries from the processed raw milk brought in by dairy farmers.
a terrific reference for milk operations and related equipment, particularly in the northeast, is the four volume paperback set: 'Railway Milk Cars'.