Tunnel boring machines are a common tool in deep tunnel construction. I am curious if they have ever been mass produced in specific types. What are the possible problems in (a possible) mass production of such machines?
One of the main problem with tunnel boring machines is they are required for a specific tunneling job: a certain sewer tunnel, a one off underground rail tunnel, etc. Once the job has been completed the tunnel boring machine may no longer be required. In a particular location there may not a need to continually excavate tunnels. Tunnel boring machines aren't easily mobile. Logistics are required to move them from site to site.
Tunnel boring machines are made to construct a tunnel of a specific diameter. They may also be constructed to deal with specific types of ground conditions: clay soils, sand, highly stressed rock, etc. A machine built to perform for a specific set of conditions may not be applicable to other ground conditions.
For a given set of tunneling conditions, tunnel boring machines may not be the most appropriate device to use. In near surface tunnels in sandy soils, a cut and cover approach may be more appropriate technically and economically.
Tunnel boring machine manufacturers don't want to have a yard of unsold machines waiting to be purchased. It's better for them financially to make machines as required and to the specifications of the client.