Transportation stocks have hardly budged in 2012. With a gain of around 2 percent, they were among the market’s worst performers, as shipments of everything from coal to refrigerators fell.

But nothing gets transportation stocks moving like US economic growth, which we expect to pick up in 2013, assuming Congress steers the country away from the so-called fiscal cliff. Below are three transportation stocks with promise, recently trading at substantial discounts to their historical valuations.

Norfolk Southern Corp (NYSE: NSC) is down 22 percent from its 52-week high and yields 3.2 percent.

Norfolk is one of America’s best-run railroads, operating 21,000 miles of track in the Midwest and eastern parts of the country. But close to a third of Norfolk’s revenue comes from coal, which is losing market share to natural gas and whose use in steel production has fallen due to the global economic slowdown.

Toward the end of 2012, Norfolk shares sold off sharply, as its coal shipments dropped 15 percent. As a result, third-quarter earnings fell 22 percent (to $1.26 per share) on a 7 percent decline in revenue. Full-year earnings are now estimated at $5.80 per share, lower than expected, but still a 7 percent increase from 2011.

Norfolk has been solidly profitable, even during the 2009 downturn. And we think its fortune will improve. The declines in coal shipments are likely to moderate. And since coal is generally moved in all-coal cars that act as a unit, with these cars often provided by clients, we think Norfolk can quickly adjust its operations to match demand. Also, some of the lost earnings from coal are likely to be offset by higher intermodal traffic (recently 19 percent of revenue), as well as increases in autos, agriculture, metal, chemical and forest products (each 7 percent to 13 percent of revenue).

Norfolk has consistently built up its intermodal business (containers made to go from ship to truck to train without extra handling). A recent upgrade to its Heartland Corridor, a 379-mile stretch between Virginia and Chicago, will accommodate double-stacked intermodal freight trains. This is a win-win: It lowers costs for shippers, while increasing per-train revenue by close to 33 percent.

Norfolk has also invested heavily in network upgrades, allowing it to handle more daily freight, as well as adding trains in America’s heartland to accommodate more cargo out of Canada.

Trinity Industries (NYSE: TRN) is just 8 percent off its 52-week high and yields 1.3 percent.

Trinity is the largest US manufacturer of railroad cars, as well as railcar axles and coupling devices. It is also one of the biggest lessors of railcars, and it provides management and administrative services, such as regulatory compliance and fleet optimization.

Trinity’s business is booming, thanks to growing US production of oil and gas. Revenue was up some 40 percent in 2012 and is on track to jump another 34 percent in 2013. The main driver is orders for railcars, especially tankers used to transport oil.

In the third quarter of 2012, Trinity received orders for 4,865 new railcars, bringing its backlog to 31,330 units valued at a record $3.3 billion. And the company is also selling more used cars from its leasing fleet at significantly higher profit margins.

Trinity should continue to benefit from the structural shift in the type of cargos that railroad cars are hauling, a trend that should drive new railcar orders over the next few years.

Swift Transportation Co (NYSE: SWFT) is down 33 percent from its 52-week high.

Due to the generally weak US economy, revenue and earnings have been flat. But this $3 billiona- year trucking company increased its load mileage 1 percent in 2012, with a 3 percent increase expected in 2013.

Swift is also expanding its intermodal operations, aiming to add 1,000 to 1,500 containers annually through 2015 and partnering with railroad companies for loading and unloading. For third-quarter 2012, the trucker’s intermodal traffic was up 42 percent.

Swift has fairly high long-term debt of about $1.5 billion. And this has compressed its valuation over the past year. But with only $8.3 million of its debt maturing over the next four years, Swift is well positioned to pay down debt out of its growing free cash flow. During the first nine months of 2012, Swift paid off $104 million in debt, ahead of its goal of $50 to $100 million in annual debt reduction.