U.S. railroads, Wall Street favorites for much of the past decade, are slumping into a bear market amid a three-way squeeze from plunging coal, crude-oil and grain shipments.

An index of the four largest publicly traded U.S. carriers has dropped 22 percent from its peak in November, paced by Kansas City Southern, as the companies struggle to offset the loss of volumes. They haven’t tumbled this much since 2011.