Tunnel boring machines are somewhat consumable, the cutting head and outside of the machine erodes over time and they have a service lifetime of the few years t

Tunnel boring machines are somewhat consumable, the cutting head and outside of the machine erodes over time and they have a service lifetime of the few years they spend boring the tunnel. Bertha spent a lot of time working and had to be spectacularly dug up to deal with excessive wear on the mechanisms, so by time it emerged in SLU it was at the end of its service lifetime. Sound Transit has been rebuilding and reusing their machines.