When does Amtrak assign reserved seats?
I just booked my first Amtrak ticket ever for a trip in mid-September, and the email I got says I have "1 reserved coach seat," but I don't see a seat number or anything to give me a unique seat anywhere. When, if ever, will that happen? Do I get emailed an assignment as the trip gets closer, do I get one when I present my ticket at the gate, etc.?
The distinction between a "reserved coach seat" and an "unreserved coach seat" is that they (sort of) guarantee that there will be seats for all passengers with "reserved" seats but not for passengers with "unreserved" coach seats. From the Amtrak website (bolding mostly mine):
While reserved coach seating is available on our long distance routes and many trains that travel short / medium distance routes, the train cars are different depending on where a train travels, so some of the amenities are different as well. ... Advance reservations (by train, not by individual seat) are required.*
Only a few short-distance trains have unreserved coach seating, where tickets are valid on any train unless restricted by the fare paid.
While seating is not guaranteed, unreserved coach seats have most of the features that reserved coach seats have...
The trains that have unreserved coach seats are Auburn (CA)–Sacramento–San Jose; Milwaukee–Chicago; Philadelphia—Harrisburg; and San Luis Obispo–Santa Barbara–LA–San Diego. I would guess that these lines are often used by commuters who want a little more flexibility in their train times.
As to how the boarding experience works: You get on the train, find a seat, and stow your luggage. Occasionally, the conductors will direct you to particular cars depending on your destination as you walk up to the train on the platform. Once the train is underway again, the conductors will go through the cars and ask all new passengers to show their tickets.
*Editorial note: when I wrote this answer in 2015, the Amtrak guidelines at at the time followed this sentence with, "Once you make your reservation, a seat is guaranteed." This sentence is no longer on the current version of the page, which strikes me as mildly suspicious.
The Sacramento-San Jose, California train is indeed popular with commuters (only a few trains per day go to Auburn). Not only to commuters want flexibility, Amtrak sells a 10-ride ticket at a significant discount to 10 single tickets, and holders of that ticket just board whichever train they please. I have never seen a train literally standing room only, but it could happen.
Chicago-Milwaukee (the *Hiawatha*) is also used for commuters, as well as for air travellers who like to fly to a quiet airport (General Mitchell in Milwaukee) and then hop on a train, and not worry so much about which, and for tourists from Chicago who want to catch a Cubs game at a reasonable rate (@Brewers) or just play around town. It's treated like a Metra (interurban type commuter line) similar to what @AndrewLazarus described.