In the nineteenth century, Russia chose a broader gauge. It is widely believed that the choice was made for military reasons, to prevent potential invaders from using their rail system. Others point out that no clear standard had emerged by 1842. Engineer Pavel Melnikov hired George Washington Whistler, a prominent American railroad engineer (and father of the artist James McNeill Whistler), to be a consultant on the building of Russia's first major railroad, the Moscow - St. Petersburg line. The selection of 1.5 m gauge was recommended by German and Austrian engineers, it was not the same as the 5 ft (1524 mm) gauge which was in common use in the southern United States at the time. Russia and most of the former Russian Empire, including the Baltic states, Ukraine, Belarus, the Caucasian and Central Asian republics, and Mongolia, have the official Russian measurement of 1520 mm, 4 mm narrower than 5 ft (1524 mm), though rolling stock of both gauges is interchangeable in practice.