keeping a certain allowance between the right and left rails (movable
clearance), as shown in Fig. 6. This makes it necessary to measure
the rail position. Further, it is specified that the reference should be
located 16 mm below the rail top surface. In the Shinkansen line, an
optical rail displacement detector has been used in order to ensure
safety during high-speed run, because this device is capable of
detecting the position without bringing it in contact with the rail side.