How and and why have the railroads been replaced by trucking?

They haven't been replaced at all. In fact, shipping coast to coast with rail is about the most cost effective mode of transportation there is. In fact, several of the major trucking companies have entered into freight partnerships with the railroad to transport their trailers from hub to hub. read more

Railroads soon replaced many canals and turnpikes and by the 1870s had significantly displaced steamboats as well. The railroads were superior to these alternative modes of transportation, particularly water routes because they lowered costs in two ways. read more