Insidious trains bug: broken switch

I'm posting this just to raise awareness as it happened to me multiple times in the few days trains are out and while it's simple to fix, it may pass unnoticed for a while, damaging the performance of your rail network, and may take quite a while to find out where the problem is.

Essentially, when you build a switch, i.e. a rail splitting into two, and then later you rebuild something and remove the common rail, often enough the switch box (with the arrow) will stay in place, and when you put the common rail back, it will only connect to one of the two rails coming to the place of the former switch joint.

The result looks perfectly like a functional switch but it will not work - your trains will never use the "broken leg". But since they're intelligent, they may find a way around and still reach their destinations, just using strange and inefficient routes. Exactly that happened to me, I only noticed this problem after I observed my trains at places where they had no business.

Also, when you replace the common rail, you can't know which of the two rails it connected to - not until you see a train go through that switch.

To fix it, you need to rebuild the whole switch - remove two of the three rails and put them back in correct order. That's usually not hard. But figuring out which switches are broken this way after a while since you did your "maintenance" through observing what trains are doing may be rather time consuming.

A preemptive workaround is, when you need to replace the common rail in a switch, first remove one of the split rails. And then put this split rail back after you're done.