By Michael Conlon

CHICAGO, June 25 (Reuters) - A new round of storms dumped a half foot (15 cm) or more of rain across parts of the U.S. Midwest on Wednesday, dealing fresh trouble to a region already struggling with billions of dollars in flood damage.

The bad news came as a key farm group estimated U.S. crop damage this year had hit $8 billion nationwide, most of it in the key Midwest growing areas of the world's biggest grain and food exporter amid the worst flooding in 15 years.

The new storms that soaked Missouri on Wednesday closed roads and sent smaller streams out of their banks, pushing more water into the basin that feeds both the Mississippi and Missouri Rivers, just as the Mississippi was cresting or about to in areas upstream from St. Louis.

A new flood warning was issued for parts of the Missouri River in parts of its namesake state from Jefferson City east to St. Louis. Lack of flood waters on the Missouri has been one of the key reasons this month's flooding was rated not as bad yet as the record flooding levels seen in 1993.

The Midwest storms and torrential rains have killed 24 people and caused billions of dollars in damage since late May. More than 38,000 people have been displaced, mostly in Iowa.

A railroad bridge leading to a Tyson Foods Inc <TSN.N> pork plant near Columbus Junction, Iowa, collapsed late on Tuesday, injuring one rail worker, local officials said. A locomotive and two rail cars fell into the still-flooded Iowa River.

The accident occurred when the locomotive was moving four tank cars filled with water off the bridge, said Steven Kulm, spokesman for the Federal Railroad Administration. The cars had been placed on the bridge to stabilize it during flooding.

The U.S. Army Corps of Engineers office in St. Louis said it was keeping a close eye on the new storms, which could cause the major rivers to rise or at least slow their fall, further stressing water-logged levees.

A battle continued along one of those levees on the swollen Mississippi River near Winfield, Missouri, north of St. Louis, where a failure could flood dozens of homes and a stretch of farm land.

Fears that up to 5 million acres (2 mln hectares) of corn and soybeans have been lost due to the flooding sent corn and livestock prices to record highs last week. Chicago Board of Trade corn for July 2009 delivery was trading at $7.81-1/2 a bushel on Wednesday, more than double the 40-year average.

Fears about the short supplies of basic food and feedstuffs have ignited more alarms about rising world food inflation even as oil and energy prices also set records.

Since late May when heavy rains hit Missouri, Illinois, Iowa, Indiana and Wisconsin, about three dozen levees have failed along the Mississippi River, covering rich and valuable bottom land and emerging crops with muddy water.

Federal disaster officials said they have handed out more than 13 million sandbags in the region, a quantity that if placed end-to-end would stretch from San Francisco to Washington, D.C.

The American Farm Bureau Federation, which issued the $8 billion national crop damage estimate, said Iowa alone had suffered $4 billion in agricultural losses. Illinois had $1.3 billion, Missouri $900 million, Indiana $500 million and Nebraska $500 million, with another $1 billion in losses expected in other states with water problems. (Additional reporting by Erin Zureick, K.T. Arasu and Lisa Shumaker; editing by David Wiessler)

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