Let's say the larger TBM can remove 7/4 the debris in the same amount of time. That means it uses 7/4 more carts or has 7/4 bigger carts and uses 7/4 more energ

Let's say the larger TBM can remove 7/4 the debris in the same amount of time. That means it uses 7/4 more carts or has 7/4 bigger carts and uses 7/4 more energy to power those carts. It places 7/4 more wear on the rails and has a higher probability of increased maintenance costs on those rails, let alone the TBM itself. All of this to make the same forward progress in the same amount of time. Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. You don't get to pretend that larger material removal rates are free. You don't get to pretend that the 7/4 more cutting wheels don't cost more money to replace. The 7/4 more energy to run the TBM isn't free. Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less.