Rail survey unit

An apparatus for measuring relative lateral position profile of an elongated rail at a series of discrete longitudinal locations, comprising:

• an elongated, rigid housing dispersed substantially parallel to the rail;
• a first support bearing, secured to the housing, for locally maintaining the housing a fixed distance from the guide rail;
• a second support bearing, spaced apart from the first bearing and secured to the housing, for locally maintaining the housing a fixed distance from the guide rail;
• a first lateral position sensor secured to the housing and spaced apart from the first and second bearings, for measuring the local lateral displacement between the housing and the rail;
• a longitudinal position sensor secured to the housing for measuring the longitudinal displacement of the housing along the rail in terms of a defined unit of length;
• and
• wherein the spacing between each of the first bearing, the second bearing and the first lateral sensor, is an integral multiple of said unit of length.