As a polar vortex snarled North America's railroads and upended freight flows this winter, everyone from agricultural giant Cargill to Dow Chemical rushed to secure the next-best form of hauling goods: trucks. In what trucking executives described as an unprecedented bidding frenzy, spot market rates surged by as much as 20 percent to record highs in the first three months of 2014 as shippers sought to minimize sometimes weeks-long delays in rail service. "We've received letters from CEOs from major chemical companies asking us if we can do more," said Bill Marchbank, vice president of operations at Houston-based Trimac Transportation, a leading bulk shipper of liquid chemicals and industrial minerals.