This is the answer, when the railway line was 
    built, the trains that ran on them were pulled
  by steam engines. The method of construction
     was known as cut and cover, where a cutting was
     dug, the track put down and a roof put over to 
   create a tunnel. Because of the steam engines
     there had to be places on the route of the line
         for the engines to let steam off into the open air.