Indiana attributes rail rolling stock to its tax base ac- cording to the ratio of track miles in Indiana to trackage in the nation as a whole. For example, if a given carline dispatches cars over 200 miles of track in Indiana, and 5,000 miles of track in its entire operations, then the state attributes 4 percent of its rolling stock to Indiana for tax pur- poses. The formula for airplanes is different, using minutes on the ground rather than miles of track. The carlines tendered an affidavit by an expert who believes that In- diana could estimate the average number of cars present in the state on any given day more accurately using a different formula. Making a precise count of cars on a date certain would be a superhuman task and would invite the carlines to play a shell game with the states.
Estimation is essential, as the carlines' own expert agrees.
Differences are merely inevitable.