Better question: What's so great about hubs?
Chicago, New York and Los Angeles just happen to be the three largest metro areas in the nation. Washington is about #6. Each is a gateway for every form of transportation simply because of the high volume of origination/destination traffic these markets generate.
The hub is a fairly recent invention, circa 1978. It reflects the economics of airline travel in the post-deregulation period and the composition of the post-deregulation fleet. The idea was to collect passengers from hither and yon as cheaply as possible, pack big jets as full as possible and fly them on segments as long as possible. Because hub-and-spoke operations don't depend on O/D traffic, you can put them anywhere there's sufficient runway and terminal capacity. That's why Northwest has a hub in Memphis, Delta in Salt Lake, US Airways in Charlotte.
But trains are not planes.
Any train, particularly a long-distance train, functions much like a hub all by itself: It collects passengers from a variety of locations and distributes them to a variety of locations. Trains can function this way because the cost of a train stop is relatively small. Hub-and-spoke operation reflects the high cost of takeoff and landings.
And even in the airline business, the hub as we've come to understand it is about as cutting-edge as that other great traffic aggregator, the
regional shopping mall. Southwest NEVER bought into the hub-and-spoke model; its service has always been pooled around large O/D sources like Houston. (And by the way, that's why it's coming to Philly, apparently in force.) US has
cut its Pittsburgh hub way back, and American has done the same in St. Louis. Quite a few cities that were once hubs (Syracuse, Dayton, Nashville, Raleigh-Durham) aren't anymore. And then there's the regional jet, a class of commercial airfcraft that didn't exist when hub-and-spoke networks were first mapped out. Cheaper to fly and easier to fill, the RJ has a whole different set of economies of scale, so it's forcing a serious rethink of the network carriers' network structure. Call it the Kohl's or Best Buy of air travel.
Does Amtrak have hubs? Not in the airline sense. Does Amtrak need hubs? No.