“ The terms of that act were such that its application depended, first, upon the carrier being engaged in interstate commerce by railroad, and, second, upon the use of the car in moving interstate traffic. ”

“ In 1908, after the case had been twice tried without any decisive result, the plaintiff sought to amend his petition by charging that the cars were used in moving interstate traffic, but the application was denied, the period of limitation having expired in the meantime. ”