Hey, thanks for the great mod!
In order to make it work to correctly read blueprints with rails the following modification is necessary:

                for _, item in pairs(blueprint_data) do
                    if item.name == "curved-rail" or item.name == "straight-rail" then
                        if item.name == "curved-rail" then
                            blueprint_bom["rail"] = (blueprint_bom["rail"] or 0) + 4
                        else
                            blueprint_bom["rail"] = (blueprint_bom["rail"] or 0) + 1
                        end
                    else
                        blueprint_bom[item.name] = (blueprint_bom[item.name] or 0) + 1
                    end
                end

Hey,
thanks for the report.
Where did you encounter this? Was there a change on the rail parts?
Either way, I'll check and implement that as soon as possible!

I tried to "import" a blueprint into a constant combinator and I wondered why everything was imported but not rails. Seems like rails are stored in a blueprint as curved-rail and straight-rail (which does not exist as items) and curved-rail is really 4 rails so there was also some arithmetic needed to get the right number of rails out of the blueprint. I guess its not a common usecase so noone really noticed.

I had no time to investigate yesterday, but the target should be irrelevant for this…

More interesting is the source.
Did you import the blueprint from the library or was it lying in the inventory?

I think I tried both.

After some testing I think it only was an issue when importing from library.
Thanks again for the report - and of course for the patch!

I added the code and uploaded a fixed version.