Improve consistency of train path preview

**Is your feature request related to a problem? Please describe.**

Currently, selecting to build factories will show a train path preview, showing how the factory can link up with existing cities.

However, for some unknown reason, sometimes the actual railway generated does not follow the preview.

**Describe the solution you'd like**

Improve the consistency of the path generation:
- make the preview really applicable to actual railway path generation
- clarify the rules of railway connection (nearest connectable features?)

**Describe alternatives you've considered**

N/A

**Additional context**

N/A

**Assignment**

- [ ] I'd like to be assigned to this issue and work on it myself