A metre-gauge railway track has a width of exactly 1 metre, which equals 1000 mm.
Metre gauge was widely used across India in the 19th and early 20th centuries because it was cheaper to build than broad gauge, especially in regions with lower traffic. Over time, most metre-gauge lines have been converted to broad gauge under a programme called Project Unigauge, which aims to bring uniformity across the Indian Railways network.
Broad gauge remains the standard at 1676 mm, narrow gauge is 762 mm, and metre gauge falls in between at 1000 mm.