As I see it there's a couple of big differences between the previous requirement of copper cable and the new requirement of rails.

1. Copper cable had 1 plate -> 2 products while rail have 3 plate/raw -> 2 products. Additionally rails are not an intermediate product and can thus not be boosted by ...

I like this idea and was thinking of submitting it here myself. Especially the relay chest. It is also useful when you what to store a specific goods in a specific area. Eg. yellow belts should be upgraded if we have a storage (so the should be stored in a chest by the upgrade assembly). But still ...