>> "(personal opinion: there are none coming)"
Former tunnel boring engineer here.
There is a ton of room for innovation around tunnel boring machines built by the major manufacturers (primarily Herrenknecht and Robbins), around reliability, durability, ease of serviceability etc. I cannot emphasize enough how modern TBM's require an unbelievable amount of engineering attention, repair labor, spare parts infrastructure etc, similar to many super-early-stage fragile prototype technologies. Unfortunately TBM's are no longer early stage, but for some reason the technology is frozen at just-good-enough-to-barely-work."
However I don't know if the economics will work out to fix any of these. Here are a couple of the big problems:
* Most TBM's are semi- or fully-customized for a single job. This raises machine costs. It'd be better if there were only a small range of small-medium-large TBM's that work ~everywhere.
* Most TBM's are fully assembled in the factory, smoketested aka turned on to make sure they work, disassembled and shipped to the job, then reassembled and used. This is not efficient, surely we can figure out a better way.
* Most TBM's are entombed aka thrown away at the end of the job, because getting them out of the hole is expensive and difficult.
* Changing the cutterheads is labor intensive and dangerous and requires highly trained very expensive humans, and it's slow. While you change the cutterheads, your billion dollar toy is sitting there doing nothing.
* TBM architecture is highly dependent on geology. A slurry faced TBM that works in mixed soils is a totally different beast from a hard-rock TBM. It would be cool to have one machine that works in many geologies, perhaps with minimal or automated modifications.
* TBM's require lots of care and feeding from a small army of humans. This raises job costs.
* Topside support infrastructure such as slurry plants and ground freezing machinery comes from different vendors, often even from different countries. E.g. it's common to buy your topside slurry plant from the MS company, in France, while your ground freeze vendor might be Tachibana from Japan. Often each subsystem's engineers on site literally don't even speak a common language. Hilarity predictably ensues. Vertical integration would pay huge dividends here.
Ideally Elon can mass-produce TBM's that just work out of the box for most jobs, and that are easier to work on. Then we can laugh him out of town for his stupid "put cars in tunnels" ideas and use his miracle machines to build sensible train tunnels.