Lance Fritz, chief executive of Union Pacific, said that movements to and from Mexico in the first quarter grew 6 per cent for the company, which covers much of America west of the Mississippi River with its network.

The traffic now accounts for 12 per cent of all UP’s business. It could be vulnerable to any renegotiation of the North American Free Trade Agreement, signed in 1994, which has encouraged a surge in trade between the US, Canada and Mexico.

Many of UP’s other business areas are suffering because of slumping demand to move coal, reduced oil and gas drilling activity and lacklustre global trade growth. Overall traffic declined 8 per cent year-on-year in the first quarter.

Donald Trump, the presumptive Republican presidential nominee, has repeatedly called Nafta “a disaster,” vowing to scrap the deal.

Mr Fritz joked in response to a question about Mr Trump’s proposal to build a wall on the Mexican border: “As long as it has portals for rail traffic”.

He otherwise declined to comment on specific candidates, saying only that UP wanted a president who cared deeply about the issues that were important to the railroad.

None of the remaining potential candidates for the White House — Mr Trump, and the Democratic contenders, Bernie Sanders and Hillary Clinton — currently supports the Trans-Pacific Partnership trade agreement with Asian countries that is the US’s biggest pending trade deal. Mrs Clinton supported it when secretary of state from 2009 until 2013, however.

Mr Fritz rejected suggestions that he was mainly criticising Mr Trump.

“I’m concerned I don’t hear any candidate right now talking about free and open trade,” he said. “I’d like to hear one of them speak to and be more positive about Nafta. From 1994 to today, US trade to Mexico is up fourfold and Mexico trade with the US is up even more.”

Mr Fritz’s criticism of the candidates comes as UP wrestles with a sharp downturn in traffic that has abruptly ended several years of rapid growth following the 2008-09 Great Recession.

Movements of coal, 12 per cent of revenue, were down 34 per cent in the first quarter over the first three months of 2015. Intermodal traffic, as movement of shipping containers and truck trailers is known, was down 3 per cent in the first quarter. Mr Fritz said that this had worsened since the first-quarter report.

Copyright The Financial Times Limited 2016. All rights reserved. You may share using our article tools.