An animated video showing the growth of London's rail system. <br /> 
Notice that almost all lines through central London (i.e., tube 
lines) were built by 1915, and none were built from 1915 to 1960. <br/> 
Also notice that sometimes lines disappear.  For instance, a line on the 
east side of London disappears in 1995, only to reappear in a slightly 
different form four years later.