You're right, it was screens, at first. They figured out that theft had become a much bigger problem than vandalism (or railfans and transients taking rides in the chevy's, playing the radio, turning on the AC or heater, and arriving out of fuel at Valla). The "closed cars" were introduced pretty fast, and as they did that you'd only see an occasional "open" bi-level with larger "commercial sized" trucks.

One of the most fascinating places I ever worked was Tweedy, firing for "Choppy" Gryde, on the daylight job that made the afternoon spot in the GM plant (South Gate). We'd switch the auto parts cars to position the cut so that each car of whatever was needed was opposite the spot on the "line" were it would be used to put the car together. We'd pull the empties and the end of the day shift and set it over to the yard in the middle of Alameda, then pickup our pre switched cut, and spotted the plant for the PM shift.

Tweedy was one of he first places we used radios for the "close work" making these precise spots, and a radio failure, had caused the night job to shove a car over the "end" and derailed it in the plant, one day. That day we had to pull the cut, except the car on the ground, go back in, to pull it back up, and we got to go about 25 car lengths into the "bowels" of the plant building, passing the stopped line, with cars in various stages of assembly, which was a real education, on the miracle of turning a pile of parts into a 1974 Pontiac they'd drive out the other end and into the lot, to be loaded out on tri-levels.

Nowadays, there's only a big hole under Tweedy Blvd. where trains hauling thousands of containers, run up and down the Alameda Corridor. My sister in-law's great gramps Phineas Banning would be truly amazed at his original port of "Wilmington" and his first and LA's first RR, the LA and San Pedro.