Canadian National Railway Co. declined to increase its offer for Kansas City Southern after a monthslong takeover battle, ceding to Canadian Pacific Railway Ltd. a prize that would create the first railroad spanning the U.S., Canada and Mexico.

CSX Corp. and Hunter Harrison have discussed a three-year contract for the railroad veteran to take over as CEO, but the deal has bogged down over details on how to support his plan to turn around the company, according to a person familiar with the situation.