Track gauge in Italy
Historically, Italy had two unusual dominant track gauges which were legally defined. The gauge of 1,445 mm (4 ft 8 7⁄8 in) was used for the national Italian rail network and was very similar to the 1,435 mm (4 ft 8 1⁄2 in) standard gauge. Since the 1930s the 1,435 mm gauge was adopted and gradually replaced the 1,445 mm track gauge. A few isolated 1,445 mm gauge networks survive.
The other gauge, a narrow gauge, was defined at 950 mm (3 ft 1 3⁄8 in) and is very similar to 1,000 mm (3 ft 3 3⁄8 in) or metre gauge and was thus called "Italian metre gauge".
Italian law defined its track gauges from the centres of each rail, rather than the inside edges of the rails, giving some unusual measurements. According to the law of 28 July 1879, the only legal gauges in Italy were 1,500 mm (4 ft 11 1⁄16 in), 1,000 mm (3 ft 3 3⁄8 in) and measured to the middle of the rail, which corresponds to 1,445 mm (4 ft 8 7⁄8 in) and 950 mm (3 ft 1 3⁄8 in) inside the rail.
The middle size has 1,000 mm between the centres of the rails, which explains why it is called Italian metre gauge, in spite of it in fact being 950 mm in gauge when measured from the inside of the rails in the way gauges usually are measured in other countries.
A disadvantage of measuring from the centre of the rail is that the width of the rail varies, affecting the gauge. It is easier and more reliable to measure from the inner edges of the rails.
...
Wikipedia