Washington (Platts)--3Jan2013/436 pm EST/2136 GMT

US railroads in 2012 hauled less coal, by far the largest commodity to travel by rail, but made gains in other commodity groups, such as petroleum products, which saw growth of more than 46%, according to annual traffic data released Thursday by the Washington-based Association of American Railroads.

Overall, US traffic, not including intermodal traffic, fell to 14,682,819 carloads in 2012 from 15,159,141 carloads in 2011, a 3.1% decline.

Intermodal units, mainly shipping containers, rose to 12,267,336 units in 2012 from 11,892,418 units in 2011, a 3.2% increase.

For the year, the nation's railroads hauled 6,025,619 carloads of coal, compared with 6,751,876 in 2011, a decline of 10.8%.

The worldwide economic slowdown is widely cited for the drop in US coal traffic, along with a warmer-than-normal winter in early 2012 coupled with falling natural gas prices, which has prompted some power companies to switch from coal to natural gas as a baseload fuel.

"Coal and grain typically account for around half of U.S. rail carloads, so when they're down, chances are good that overall rail carloads are down too, as we saw in 2012," said AAR Senior Vice President John Gray in a statement accompanying the rail traffic figures.

"That said, a number of key rail carload categories showed solid improvement in 2012, including categories like autos and lumber that are most highly correlated with economic growth," Gray added.

The petroleum products category, which includes shipments of crude oil from drilling areas such as Bakken field in North Dakota, grew to 540,563 carloads in 2012 from 369,569 carloads in 2011, a 46.3% increase.

Carloads of crushed stone, sand and gravel also rose during the year, primarily due to increased demand for frac sand, increasing to 972,051 carloads in 2012 from 901,039 carloads in 2011, a 7.9% increase.

Other commodity groups with grew in 2012 included lumber and wood products, which grew to 161,858 carloads in 2012 from 143,199 carloads in 2011, a 13% increase; and automobiles and parts, which grew to 806,835 carloads in 2012 from 692,614 carloads in 2011, a 16.5% increase.

Shipments of metals grew during the year, to 542,675 carloads in 2012 from 531,735 carloads in 2011, a 2.1% increase.

However, metallic ores carloads fell to 374,359 carloads in 2012 from 396,780 in 2011, a 5.7% decrease.

Iron and steel scrap also fell, to 229,441 carloads in 2012 from 240,355 carloads in 2011, a 4.5% decrease.

Grain carloads also fell, to 1,017,377 carloads in 2012 from 1,123,666 carloads in 2011, a 9.5% decline.

Chemicals dropped to 1,536,194 carloads in 2012 from 1,548,109 carloads in 2011, a 0.8% decline.

--Andrew Moore, andrew_moore@platts.com --Edited by Katharine Fraser, katharine_fraser@platts.com