What is a chord? A geometric one not a musical one
A chord is a line drawn across a circle that creates an arc or segment.
But when it comes to the railway it can also refer to the length the track is broken up into for surveying purposes. The standard chord length is 20m, with half chords at 10m. This means measurements will be taken every 10m.
For more on on Hallade surveying and railway engineering, check out the channel