I have a fun story. When I was 18, I worked at the railroad yards in Indiana the summer before college. This was 1976. I was mowing grass and stopped at this shack near the hump to use their restroom. The hump is how the train cars are sorted. A locomotive pushes the cars up the hump and then the cars are cataloged and coast down the hill to a giant mass of tracks where they are switched to new trains being built. It's a hub for sorting the train cars to their next destination. Anyway, this shack I found housed an analog computer that used RADAR to read the speed of the railroad cars coasting down the hump and apply brakes accordingly. I walked in and first saw a large room full of wet cells (batteries). The main room was maybe 20x30. A guy was sitting at a desk at one end. The other end had a small meter reading miles per hour in front of windows looking at the hump and the cars rolling down. On both sides of this room were banks and banks of chassis full of vacuum tubes- hundreds of tubes. This giant room held the computer whose one task was to control the speed of the train cars. I commented to this guy that he must stay busy changing out vacuum tubes. He said he hadn't had to change a tube in 15 years. The tubes were fed a constant DC voltage from the wet cells and apparently had little stress.
One of the coolest things I have seen. The railroad yards had some fascinating technology back in the day.