TORONTO, May 27 (Reuters) - Canadian Pacific Railway aims to have the best operating ratio among North America's biggest railroads in 18 months time, the chief executive of Canada's No. 2 rail carrier said on Monday. "We're going to cross and be below all of the railroads on operating ratio," Chief Executive Officer Hunter Harrison said during an interview on BNN Television. "It's about 18 months away." Operating ratio measures productivity by tallying how much revenue is required to maintain operations. The lower the number, the better.