NJ, 2012 sleepers to the mile equates to 24 sleepers to the "length", a length being 60'0" or 20 yards, the length of a single piece of jointed rail. that wouldn't be enough support for modern high speed railways, where 28 or 30 sleepers to the length is now the norm (depending on the track category, which is based on speed and weight of traffic). today, railway staff are told to walk "ballast to ballast" because concrete sleepers can be slippery if wet, frost covered or contaminated.
just for interest, this is how track is laid these days....
https://mccullochrail.com/the-machines/flass/