Sept 17 (Reuters) - Canadian National Railway Co (CNR.TO) said on Friday it would resume a previously approved share buyback, days after walking away from its $29.6-billion deal for U.S. railroad operator Kansas City Southern (KSU.N). The largest Canadian railroad expects to complete the remaining C$1.1 billion ($869.02 million) of share repurchases by the end of January next year, it said in a statement. The company, under fire from some investors for its failed bid to buy Kansas City Southern, said it was targeting C$700 million of operating income improvements in 2022 by increasing labor productivity and reviewing its non-rail businesses. It reaffirmed its 2021 targets of a double-digit increase in adjusted earnings per share and free cash flow. Canadian National"s abandonment of its approach for Kansas City Southern allowed rival Canadian Pacific Railway Ltd (CP.TO) to cement a deal to buy the U.S. peer and create the first direct railway linking Canada, the United States and Mexico. read more ($1 = 1.2658 Canadian dollars)