There are at least 3 generations in use: Purely mechanical ones, relay based (which is digital), semi-conductor based computers of various age. I don't think an

There are at least 3 generations in use: Purely mechanical ones, relay based (which is digital), semi-conductor based computers of various age. I don't think analog computers have been used in any phase. I would not be surprised if purely mechanical ones achieve higher reliability than some generations of digital ones. But many small signal boxes are vulnerable if there is not enough staff, which has been a common problem for a couple of years. Naturally mechanical signal boxes are very limited in their range.