This notation is an old one. The 'N' or 'M' in this stands for the length of a rail in yards. The additional number specified represents the excess of the number of sleepers over the number of yards for a rail. E.g., 'N+3' for 11-yard (33') rails indicates 14 sleepers (11 + 3) for each rail. This was a convenient formulation, especially when rails were manufactured to sizes of 11, 12, or 14 yards. Before the days of mechanized track laying, it was common to see track laid where the sleeper density was not uniform, with some bunching up of sleepers towards the end of each rail, with adjacent sleepers at the ends of neighbouring rails being less than a foot apart in some cases.
Source - IFRCA.org
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