Interestingly my dad worked for the company that supplied the air compressors that the tunnel drilling machines used. Two in each machine, both able to do the job by themselves, which was a good thing, as the sales guys hadn't had the engineering guys take a close look at where the compressors were to be mounted.
The air compressors were totally innacessable once the drilling machine started, and had to work for over a year non-stop. They only realised when they sent a guy to service them. Cue much frantic looking at the contract, and then much crossing of fingers when the size of penalty clause bacame apparant.
Bottom line was if both compressors failed, thay would have had to dismantle the whole drilling machine.