ns_LogoU.S. railroad operator Norfolk Southern Corp all but rejected a $28.4 billion acquisition offer by Canadian Pacific Railway Ltd on Tuesday, calling it “low-premium” and warning it would face significant regulatory hurdles.

While Norfolk Southern said it would carefully evaluate the offer, its sour response represents a setback to Canadian Pacific as well as its largest shareholder, William Ackman’s activist hedge fund Pershing Square Capital Management LP.

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