Before transistors, relays were widely used in telephony switches. They were much slower, but much more reliable than vacuum tubes.

This 1951 booklet, "NO. 5 CROSSBAR" from Bell Labs describes the bad-ass mainframe of relays. Lots of photos and diagrams.

https://drive.google.com/drive/folders/1UBnLJIzYjpGu1401I6F3lDa9fWiXSBu5?usp=sharing