Had the same idea :-). To revive freight tunnels. With the advancements in self driving tech and tunneling tech this would seem feasible.

Amazon could finance them and then rent capacity it out to other companies.

A pilot perhaps in Chicago by reviving the old freights tunnels - https://goo.gl/ErJQFx

The Swiss were/are considering them too - https://goo.gl/PNMRrE