The 99 tunnel blockage would have been resolved far faster if the machine had such high tech features as the reverse gear[1] which would have alleviated the need to spend 2 years digging a massive pit to get repair it when it ran into a steel pipe.

[1] I have heard that the machine supported a reverse gear but Seattle elected not to include its use in the contract because it added a few million in cost. :/ I don’t know if this is actually true though.

From what I've heard from a relative of mine who works in the industry, there were also a few features requested for that machine which haven't been included in another boring machine of nearly that size, so very experimental. They also said that Hitachi Zosen ultimately won the bid because all the other competing companies bowed out because they considered it to be far too risky, and it seems that they were ultimately right in doing so.

How would you reverse a tunnel boring machine? Sure, you can take part of it out the back but the cutting head is usually one piece and is at least the size of the outer diameter of the tunnel. You couldn’t fit it back through - you’d only be able to go as far as you hadn’t lined, which is usually only about a metre long at most...

Detacheable and separable head? Don’t honestly know. I’d heard that it was an option for Bertha that was declined. Maybe there was lots of misinformation because I heard similar things several places.

Its not just the boring speed itself. Its also the cost of equipment. Public transit and home prices in a lot of countries could be lower if cheaper Tunnel boring machines would be available.

For too long that part has been fiefdom of big earth mover companies.

> Boring speed is not the main limiting factor for those projects.

It's equivalent to measuring software developer productivity in lines of code.