I'm not really sure how to describe this more clearly, it's a pretty simple thing.

1. On a planet in remote view: Place a common Quality Module 3 in a machine. Then place a legendary Quality Module 3 in the same slot. The common module will not be removed from the machine until a bot replaces it ...

Normally, when you replace a module in a building with a module of higher quality, the original module will only be replaced once a construction bot arrives with the new module, and if the new module is not available in logistic network the original module will continue functioning indefinitely. I ...

I've described the problem below with fluid wagons, but the same behaviour happens with a similar train with cargo wagons.

I have the following blueprint which includes:
A locomotive
A train group for that locomotive
A request for fuel for that locomotive
2 fluid wagons
No tracks, I paste ...