On the London Underground, to minimise the risk of subsidence, the tunnel alignments largely followed the roads on the surface and avoided passing under buildings. If a road was too narrow to allow the construction of side-by-side tunnels, they would be aligned one above the other, so that a number of stations have platforms at different levels. Moreover is very useful if the line branches from the station, so diverting tunnels or tracks do not intersect each other.
