Try not to
Think about building this train like any other train has ever been constructed. This will be a whole new animal. It will not be so far underwater that it will be crushed, but it will be deep enough to avoid surface disturbances. It will make use of simple bounacy compensators to maintain equalibrium.
The major problem will be economics and terrorism. How many people can they move with one tunnellvision even if it has two trains moving opposite directions 24/7. Will people pay? What will the ROI be even with full ridership and will it come close to the R.O.I? Sticky widget.
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Money, money, money, makes the world go around.
