There is already a great deal of freight moving by rail, including intermodal shipments (trailers that stack on a set of wheels and then get pulled by a regular over the road semi) and the cost per mile is substantially lower than using a truck.  BUT it takes 1-3 days longer in transit and is subject to congestion at the relatively few unloading points.  It also is more susceptible to service disruptions (lots more roads than rails in the country) and can cause product damage (whole lot of shaking goin' on).

I personally think that higher fuel costs may lead to a reversal of the mega-warehouse model.  More manufacturing sites in more places with smaller output equals less transit.  With diesel at $2.79/ gal it makes sense to ship things from China to Florida.  How about at $4?  $8?  $10?  At some point the financial benefits of mass production get eaten up by transit times and transportation costs.