If you just look at the signals as "a little red light that tells the driver to stop", then its not that complex. However in the rail world signals encompasses

If you just look at the signals as "a little red light that tells the driver to stop", then its not that complex. However in the rail world signals encompasses much more than that red light. There's sensors on the track which detect when a train enters a section of the line and when it leaves, and on modern systems there's communication with the train itself to force the brakes on if the driver runs a red light, making it near impossible to enter a section with a train already in it. All those parts need to communicate with each other, and they need to be ridiculously reliable, and capable of self-reporting when they fail, because in the worst case a failure in any one of those components results in a few hundred people dying in a dark underground tunnel.