Yes, I should have said "projects" rather than "systems".

Its when you need to coordinate it lots of bits. if you can modularise it its fine. If you have lots of small parts your replicate repeatedly its fine. So Each station is separate, they you lots of common components, trains come off a production line etc.

However building a single complex system as one project is the problem. A long distance railway line, especially if going through populated areas or difficult terrain that imposes constraints, for example.