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The Canadian Pacific Railway recently approached CSX, the big Florida-based rail line, about a merger that would create a company worth more than $60 billion, people briefed on the matter said on Sunday.

While the two companies have begun to discuss the possibility of a transaction, CSX was cool to the idea, and it is too early to tell whether they will pursue one, these people said.

If completed, a deal would unite two of the biggest railroad operators at a time when rail traffic in North America is soaring because of the energy boom, snarling Amtrak traffic and tying up other freight.

It could also provide a big victory for the activist investor William A. Ackman, who joined Canadian Pacific’s board after a contentious proxy fight and whose hedge fund, Pershing Square Capital Management, has a big stake in the company.

Canadian Pacific, with a market value of about $32.5 billion, has rail lines that stretch across Canada and into the United States. CSX has a market value of about $30 billion and controls a network of lines throughout the Eastern United States.

With minimal geographic overlap, the two companies would have a huge combined footprint. But there are potential obstacles to a deal.

CSX shareholders would most likely expect a significant premium for their shares. And consolidation is difficult in the railroad industry, with the United States Surface Transportation Board, which oversees the industry, looking warily on combinations.

Nonetheless, Canadian Pacific appears emboldened after its shares have risen 25 percent this year.

Canadian Pacific has undergone a turnaround over the last two years under the leadership of E. Hunter Harrison, a longtime rail industry executive who was brought in by Mr. Ackman. Under his tenure, the railroad has focused on trimming costs and improving efficiency. During the same period, Canadian Pacific has taken market share from its rival Canadian National and has begun to compete with the trucking industry in the market for delivering goods.

Much of the North American railroad network has been weighed down by mismanagement, and part of the logic behind uniting Canadian Pacific with CSX is that a combined company would help relieve congestion by streamlining certain crucial routes, people briefed on the deal say. Because Canadian Pacific mainly runs routes in Canada, there is little overlap with routes run by CSX or other American railroads, making it less of a concern from a regulatory perspective, a person briefed on the discussions said.

Mr. Harrison underlined the benefits of deal-making activity in the rail sector at an investor conference this month.

“Would we ever consider anything?” he said. “As I’ve said publicly before, sure. But you got to have somebody to dance with, and I don’t know anybody who wants to dance now.”

Moreover, Mr. Harrison said that deal-making at this scale was difficult. “My experience in the past, and I’ve had too much of it, it’s more about social and egos than it is true bottom-line value to the shareholder,” he said.

He said that consolidation would help ease congestion in the United States. “M.&A. makes sense in the U.S.,” he said.

But the threat of scrutiny by the Surface Transportation Board looms large, and it is unclear whether a deal between a Canadian company and a United States company would come under national security scrutiny.

Yet Canadian Pacific is most likely to have already considered such hurdles. Linda Morgan, a member of Canadian Pacific’s board, is a former chairwoman of the Surface Transportation Board.

Mr. Ackman won control of Canadian Pacific’s board in May 2012 after waging a proxy battle against the company in a move that shook up the Canadian business establishment. He gained control after Fred Green, the company’s chairman, and four other board members unexpectedly resigned. Late last year, Mr. Ackman’s Pershing Square sold part of its stake in the company, nearly tripling its original investment. It still holds 8 percent of the company’s shares.

CSX has also been shaped by activist investors. In 2008, the Children’s Investment Fund and 3G Capital Partners won seats on the CSX board.

Canadian Pacific and CSX both declined to comment.

The Wall Street Journal reported news of the overture earlier.