Tube Expansion Achievement
Total length of rails in tunnels exceeds 100.
On any map, simply build 100 lengths of rail underground and the achievement will unlock. Note that the achievement description is incorrect as it unlocks at exactly 100. To build rail underground, press twice to access the cutaway, choose the second box from the bottom, press , . Open the Rail menu, choose Flat Rail, and place 100 pieces. They do not have to touch each other or lead anywhere, although obviously it is quicker to place long strips of rail at a time.
If you are following the road map, this can be done on the first map when you are working on Public Transport King, or in conjunction with Subway Mall Expansion.