Union Pacific, a freight rail giant, announced on Tuesday that it had reached an agreement to acquire Norfolk Southern, another large railroad, in a deal worth $85 billion.

The merger would create the United States’ first coast-to-coast rail network and span some 50,000 miles across 43 states. But the deal would put around two-fifths of rail freight in the hands of one company, raising fears that it would reduce competition in a crucial industry.

Union Pacific, which operates west of the Mississippi River, and Norfolk Southern, whose tracks are mostly east of it, said the combined company would deliver freight faster by eliminating the need to switch railroads and opening new routes.

“Railroads have been an integral part of building America since the Industrial Revolution, and this transaction is the next step in advancing the industry,” Jim Vena, chief executive of Union Pacific, said in a news release on Tuesday. The new company will be called the Union Pacific Transcontinental Railroad.

Union Pacific said it was offering a combination of cash and stock for Norfolk Southern.

“It will make the railroads more efficient,” said Tony Hatch, a veteran rail analyst at ABH Consulting. “And it adds reliability, which is the biggest rail issue.”

The combined company would have over 50,000 employees, four-fifths of whom belong to unions.

The merger announcement comes two and a half years after a Norfolk Southern train carrying hazardous materials derailed in East Palestine, Ohio, upending life in the town for months. Last year, the company fired of its chief executive for having an affair with its chief legal officer.

Federal rail regulators will have to review Union Pacific and Norfolk Southern’s merger proposal to assess whether it undermines competition in an industry already dominated by a small number of companies.

Because the companies do not operate in each other’s regions, the tie-up would not reduce choice between railroads in those areas. Still, the companies together accounted for 43 percent of all rail freight movements last year, according to an analysis of regulatory carload data by Jason Miller, professor of supply chain management at Michigan State University.

Union Pacific and Norfolk Southern are most likely betting that the Trump administration, already pushing deregulation in the rail industry, will approve the deal. On Tuesday, the companies said they hoped to complete the merger in early 2027.

Over the years, freight rail has lost business to trucking companies.

Customers have chosen trucks, even though they are more expensive, because they are quicker and more reliable. One of the big problems is that rail freight moving across the country has to be switched from one railroad to another in places like Chicago, where bottlenecks occur.

Some rail experts say a merger of Union Pacific and Norfolk Southern would create the incentives for them to invest in projects aimed at speeding up cross-country journeys. One way to do that, they say, is to route more traffic away from Chicago and through hubs like St. Louis or Memphis.

Still, the two railroads’ customers — large companies that ship coal, cars, chemicals and shipping containers filled with consumer goods — will be concerned that the combined company will use its power to charge higher rates.

“It would cut off shippers from having options for transcontinental shipping,” said Erik Peinert, an assistant professor of political science at Boston University, who has studied monopolies.

The merger would cut the number of major freight railroads in the United States to five from six. Most of the freight, however, is carried by just four companies: Union Pacific, Norfolk Southern, CSX and Berkshire Hathaway’s BNSF. If BNSF and CSX were to also combine, as some analysts expect, nearly 90 percent of rail freight would be in the hands of two companies.

A spokeswoman for BNSF declined to comment but pointed to Warren Buffett’s denial last week that Berkshire had hired bankers from Goldman Sachs to assess a deal with CSX. On a call with analysts last week, CSX’s chief executive, Joseph Hinrichs, declined to comment on merger speculation.

The Surface Transportation Board, the regulator that approves rail mergers, can require that Union Pacific and Norfolk Southern take actions aimed at limiting their influence, like giving competitors the right to run trains on their merged network.

Under its rules, the transportation board has to assess whether the merger enhances competition. The board is currently split between two Democratic appointees and two Republicans. A fifth board member, to be appointed by President Trump, may not be installed for many months.

“Creating the Union Pacific Transcontinental Railroad is overwhelmingly in the public interest and will enhance competition,” the companies said on Tuesday.

And some logistics experts said the threat to competition from rail mergers may not be as serious as it looks. Mr. Miller, the supply chain professor, said that even if the four largest railroads became two, there would still face serious competition from trucking, particularly for goods moved in containers. “They wouldn’t have much pricing power,” he said.

The merger would be a milestone in American railroading history. George Jay Gould, a powerful businessman, came close to creating a coast to coast railroad early in the last century, but the effort fell apart in the 1907 financial panic. Amtrak runs coast to coast but not all the way on its own tracks.