Digging more than a few meters for a building is not only expensive, it can be very difficult and dangerous for the buildings a few hundred meters away, depending on the nature of the soil.

There is a certain cursed railstation project in germany, where the nee railstation is build underground. Though the soil is a specific type, which sucks up any water it gets in contact with and then expands. If there is a significant leakage, we are talking about half a meter difference at ground level for the complete neighbourhood, probably very inconsistent. Building typically don’t like the ground moving that much. So you start investing billions more into the project to make it water tight and still fail to do so.

And after many years you are still not finished and the project seems to be a coup by the car lobby to discredit travel by rail.