Citation? I work in the international terminals industry, going on ten years, for the 4th largest terminal operator in the world. A couple years ago I was reporting volume to various entities, so I saw what went where on a daily basis at our terminals, one in every major port in the US.
I think you're misunderstanding me. Very few containers are unpacked on-dock, these are called on-dock CFS facilities. This is most common with USAID bulk shipped in containers (grain shipped out of Houston, for example) and a few other fringe uses.
What I was referring to is how the containers get to/from the shipping terminal to the various companies' distribution facilities, which is predominantly by truck, not by rail. Almost all major distribution facilities are located near major coastal ports and are trucked to those locations from the shipping terminal. At these facilities, the intermodal containers are stripped and sent back to the terminals for re-use, typically empty. Its generally not economical to rail containers anywhere within 75 miles of the current location. Rail is used when it can be, but its volume is low compared to cargo that leaves through the truck gates.
Also, rail links between coastal cities aren't as big of corridors as you'd expect. A ship that calls Los Angeles will also stop in Oakland and Tacoma - it is always cheaper to transport by water than by rail or truck. Even cross-country shipments to coastal locations are rare - you don't ship from China to New Jersey by dropping it off in Los Angeles and railing it cross-country, you go through the Panama Canal or the Suez Canal and dock directly. Only time-sensitive cargo (a very small portion of overall cargo) would do this, and usually by long haul truck.