The sleds are pretty cool, so long as you're using them to move people instead of cars. But if they can bring the cost of boring down by a factor of 10, that in and of itself would be a game changer, regardless of what you use the tunnel for.
It's a wonder Teslas cost as little as they do when you consider each one requires "digging up 100 million tons of earth" just for the battery. Me, I can't even get 10 million tons dug for less than $100k!
Those darn fake environmentalists. They're right up there with the scotsmen of the "no true" variety.
Pick a poison gas, buying and operating an all-electric car generates less of it per mile than a gasoline car, even if your electricity is all-coal. It also enables even greater poison control by using renewable electricity sources.
The right solution is as you state public transit, which is more space efficient at moving people than cars.