BILLINGS, Mont. - A candy industry magnate who owns a sprawling southeastern Montana ranch is pushing state lawmakers to adopt legislation that could undermine a proposed $341 million coal railroad through his property.

The long-delayed Tongue River Railroad would stretch south from Miles City to almost the Wyoming border. It would open a billion-ton coal deposit near Ashland to development and provide a new route for coal coming out of mines in Wyoming.

But the railroad faces a potential obstacle in House Bill 422. The bill would tighten eminent domain law to make it harder for private property to be taken over the owner's objections.

Recent lobbying for the bill by representatives of Forrest Mars, Jr., marks the billionaire's most high-profile attempt to date to turn back development of the vast coal and natural gas deposits in Montana's Powder River Basin.

Mars, 77, is former chief executive of Mars, Inc., the family owned candy empire based in McLean, Va. He owns the 140-square-mile Diamond Cross Ranch in Birney and has a personal fortune estimated by Forbes magazine at $12 billion.

In recent months, Mars' courtroom tangles with other sectors of the energy industry have ended in a pair of losses, including one that allowed a Wyoming natural gas company to erect a drilling rig on his ranch.

In that case, the Montana Supreme Court said state law gave Pinnacle Gas Resources the right to access Mars' ranch because the company controls the underground oil and gas leases.

With the railroad, Mars and others whose properties would be bisected by the line have taken a more direct approach - attempting to rewrite state law to make it more difficult for the project to succeed.

The eminent domain bill was drafted by a Mars lobbyist and sponsored by Democratic Rep. Jill Cohenour of East Helena. It faces opposition from some lawmakers who see the measure as subverting economic development in cash-starved southeastern Montana.

"I would hope that we do not allow rich out-of-state people who come in and buy land to be able to turn our legislative system completely upside down," said state Sen. Keith Bales.

Bales is a Republican from Otter whose district encompasses much of the route of the proposed railroad. He said his district does not include Diamond Cross.

Cohenour said she was not aware the bill was drafted by a Mars lobbyist when she agreed to sponsor it. She said she was seeking to protect the "Montana value" of private property rights - not to shield a wealthy landowner from development.

"It's a red herring saying this is a rich guy trying to buy something," she said.

Diamond Cross manager Denise Wood said it was wrong to assume Mars was looking out only for his personal interests. Wood, a Montana native, said the property is managed as a working cattle ranch with 1,300 cow-calf pairs.

Wood and others who run ranches along the Tongue River say the railroad would cut off some of their fields from irrigation. Concerns also have been raised about potential damage to the river during construction.

"It's not about Forrest's deep pockets," she said. "We're thankful he can afford some ability to fight back. A lot of individual landowners don't have that. He's trying to protect it."

Current state law sets a lax standard for using eminent domain. Property owners can go to court to negotiate how much they get for their land, but not challenge eminent domain claims directly.

The U.S. Surface Transportation Board approved the final section of the 130-mile Tongue River Railroad in October 2007. It was first proposed in 1983, but has faced repeated delays caused in part by opposition from landowners in the Tongue River valley.

Railroad developer Mike Gustafson said the line would pass through Diamond Cross for 7.5 miles and take just over 240 acres of the property.

Gustafson said the eminent domain bill comes just as the project is nearing a long-sought milestone.

In the next two months, the Department of Natural Resources and Conservation is set to release its appraisal of the Otter Creek Coal tracts. Co-owned by the state and Great Northern Properties, the tracts contain an estimated 1 billion tons of coal that Gustafson hopes his railroad will someday bring to market.