RCW 84.16.030

The president or other officer of every railroad company whose lines run in, into or through this state, shall, on or before the first day of April in each year, furnish to the department of revenue a statement, verified by the affidavit of the officer making the same, showing as to every private car company respectively, the name of the company, the class of car and the total number of miles made by each class of cars, and the total number of miles made by all cars on its lines, branches, sidings, spurs or warehouse tracks, within this state during the year ending on the thirty-first day of December next preceding.