![Image](https://forums.factorio.com/images/ext/3bec4ba95098e34cbe747647f91214b3.jpg)
Now do these steps in order:
1) enable the combinator to lock the signal. the chain signal stays green
2) place a train on the marked track. the chain signal turns blue
3) remove the train. the chain signal stays blue
4) disable the combinator to unlock the signal. the chain signal stays blue