OMAHA, Neb. (AP) — Canadian Pacific railroad continued its assault on rival Canadian National’s competing $33.7 billion bid to buy Kansas City Southern railroad Wednesday in a formal letter to regulators. Canadian Pacific urged the Surface Transportation Board to closely examine Canadian National’s offer to buy Kansas City Southern because it says that deal would hurt rail competition throughout the central United States and it could destabilize the balance between the industry’s six largest players. Meanwhile, Canadian National maintains that the bid it announced Tuesday is superior to the $25 billion cash and stock deal that Canadian Pacific announced last month.