The sub-surface lines were built by digging trenches and then covering parts of them back up with roads, buildings, etc. This technique is called cut and cover.

The sub-surface lines were built by digging trenches and then covering parts of them back up with roads, buildings, etc. This technique is called cut and cover. They're just beneath the surface and large stretches of them are actually open to the air, many were built for coal powered trains. These can be built without tunnelling. They're below-ground but not usually "under ground". The trains are rectangular shapes since that is the traditional shape for a train. The deep lines (originally called "the tube" although that now refers to the whole system) were built by tunnelling, the trains and tunnels are cylindrical and the trains just fit into them. In many cases the tunnels are 30+ metres underground.