> smaller tunnels are far cheaper to build than massive ones I'm not sure how true that is these days, tbh. As I understand it, today, a lot of the cost is car

> smaller tunnels are far cheaper to build than massive ones I'm not sure how true that is these days, tbh. As I understand it, today, a lot of the cost is care and feeding of the TBM, plus planning and rerouting of obstacles etc. Going for a _bigger_ TBM adds some cost, but not all that much cost. In particular, it is notable that, whereas traditional deep-bore metro lines were pretty much always built with two tunnels in the 3-4 meter (ie 28sqm to 50sqm) range (similar to Elon's car tunnels), most _new_ deep-bore metro lines are now being built with a single tunnel in the 9-12meter range (250sqm to 450sqm). Examples are Dublin, Barcelona, Paris. Obviously this requires removal of a lot more material, but the simplicity of a single big tunnel still seems to win out.