By Nick Carey and Sinead Carew

CHICAGO/NEW YORK, Sept 28 (Reuters) - Investors in U.S. railroad stocks, who have been punished in 2015 by an accelerating decline in high-margin coal shipments, now are pinning their long-term hopes on a resurgence in consumer spending.

Their bet is that a strengthening economy will produce enough demand that railroads will be able to replace the income lost to years of declining coal use with so-called intermodal shipping - the movement of containers stuffed with clothing, furniture and other consumer goods.

Thanks to environmental rules regulating power plant emissions, coal use has declined steadily since peaking in 2008.

So far this year, freight volumes have tumbled 9.2 percent as low energy prices encouraged utilities to switch to burning cheaper natural gas, while the strong U.S. dollar has hurt exports. The accelerated decline this year has highlighted expectations that barring regulatory changes, railroads' revenue from coal should continue to decline.

Railroad shares have plunged as a result. The Dow Jones U.S. Railroads index has pulled back 27.9 percent this year after rising 39.7 percent in 2013 and 28.9 percent in 2014. The railroads' decline has contributed to a 14-percent year-to-date decline for the broader Dow Jones transportation average.

The major publicly-traded U.S. railroads including Union Pacific Corp, CSX Corp and Norfolk Southern Corp have missed estimates or trimmed earnings forecasts, furloughed workers and parked equipment, largely as a result of the coal downturn.

"All that coal is not coming back," said Morningstar analyst Keith Schoonmaker. "The question is how much (consumer goods) can mitigate those declines."

It is an open question. Intermodal freight volumes are more than double coal and make up 47 percent of total rail traffic, according to the American Association of Railroads, but that is a low-margin business where revenue per carload is often half or less than coal.

And though intermodal shipments have risen more than 17 percent since 2008, the moderate 3.1 percent increase in intermodal freight shipments so far this year has not been nearly enough to offset coal revenue declines.

If the economy and consumer spending hit a consistent stride in 2016, that would lift intermodal shipments - perhaps enough to beat 2015 volumes. But with consumer spending on goods and services expected to rise just 2.8 percent in 2016, according to the Congressional Budget Office, it will likely take years to offset coal's decline.

"If economic growth can pick up, the rails will look like a bargain," said BB&T Capital Markets analyst Mark Levin.