In the 50's, you'd see predominately single dome tank cars like the ones Charles posted. You'd also see double and tripple domed cars on occasion. The number of domes generally corresponded with the separate compartments in each tanks car. A triple dome car, for example, might carry regular, premium, and diesel fuel, all in separate compartments in the same car. Most cars, even the ones with corporate logos, were owned by tank car companies like GATX and UTLX and leased to the corporation. By the late fifties, chemical tankers were showing up. These had single domes but were characterized by multiple valves on the dome and a walkway and handrails around the dome to allow a worker to safely load and unload the car. Since you're modeling the B&M, do a search on "milk tank cars", since they were common on the B&M and other New England railroads until the late 60's. Most tanks cars were still of riveted construction in the 50's, but some welded cars were on the rails by 1959. Really, almost any kind of tank car would have been found on the rails in the late 50's except the frameless tank cars that are commonly seen today.