system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know how to build large scale, but rare, civil engineering projects? (e.g. subways)

Melbourne (Australia) is building its first subway since the 1980's.  Building subways doesn't seem that common around the world in general.  When a project like this is undertaken, how do we find people who have expertise in building them?  Furthermore, when the project ends, how is the expertise gained in building that project kept/maintained for the next one?  Since these sort of projects are so rare, it seems hard for people to build up their experience to do each subsequent project better (as one would building multiple skyscrapers, or websites for example).

Are these projects mostly done by people doing it for the first time?  Are they informed by past successes and failures somehow?

EDIT: Thanks everyone for the fantastic responses so far!  A lot of people are focusing on the 'subway' example, which was which first prompted my question, but apparently aren't as rare as I first thought.  So a side question would be, are there any projects where maintaining knowledge and experience in building it does become a problem, simply because the projects are so uncommon?  My other thought was dams, but they seem common too.
gpt
Rarely will you find an expert in something like building a Subway. You'll find some engineers who specialize in designing tunnels, and some who design railroad systems, and people who design high-traffic spaces (like stations). They'll all work together, as well as with other people who have experience in the subparts of those tasks (electrical system design, HVAC, geologists, interior design, etc). 

All of these people can apply their skill sets to a variety of projects - it's all about gathering the right group of people.