<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by srodr:
Can someone explain this difference between a reserved and an unreserved train?</font>
In theory, on a reserved train, Amtrak only sells as many tickets as there are seats, so a seat should be available for you (where that seat may be is another question--as you pointed out, there are no assigned seats, so it's kind of like Southwest Airlines in that regard). Now, a usually reliable source told me that Amtrak does practice overselling with respect to reserved trains, at least within the Northeast Corridor, but I have no positive proof or evidence of that.
For an unreserved train, Amtrak will sell as many tickets as they can. In fact, in the markets that have unreserved trains, you can buy an unreserved ticket that is not linked to any specific train, but merely covers the passage between two points--quite like buying a commuter rail ticket. Indeed, Amtrak actually sells monthly commuter "flash" passes in some of their short-haul markets that allow the holder to board any unreserved train (within the passes validity, of course). There is no denied boarding, as the trains can run over-capacity--in other words, people will be standing (or doing whatever else people do when there are no seats available). This is frequently the case during holiday periods, specifically Thanksgiving weekend.