Santa Clara Valley Transit Authority plans to build a 55,1 foot (~17 meter) in diameter subway tunnel for the BART Silicon Valley Phase II project. BART stands for 'Bay Area Rapid Transport'.

Lengthwise it is not such a long line, about 4,7 miles. But it is a significant one for the Silicon Valley. It will cost 5.6 million dollars. Despite this cost, the expected benefits are huge:

VTA experts decided on single tube tunnel after visiting some subway projects in Spain. The original plan was for a 45 foot single tube subway. In comparison, a typical single way subway tunnel would have a diameter of 6.x meters.

But the the designers soon discovered a constraint with the  45 foot single tube design. It would require platforms for each direction of traffic to be built on different levels, something VTA did not want in order to keep emergency evacuations simple. At the end, it was decided on the 55,1 foot diameter bore. That is a whopping 50% bigger bore! Wide enough to host a 4 lane highway. Thus it makes it possible to build a 24 ft. center platform with concourse above inside tunnel.

This is the timeline of the project

You can access the project site here:

https://www.vta.org/projects/bart-sv/phase-ii

You can read the VTA BART Phase II Board Workshop Presentation September 20, 2019. It is a PDF file.

https://www.vta.org/sites/default/files/2019-10/2019-09-20%20VTA%20Board%20Workshop%20Presentation_2SlidesPerPage_Reduced.pdf