Interesting. I was told that they no longer issued accommodation-only tickets. In fact, I believe I even asked for such a ticket a couple of times in the relatively recent (i.e. past two years or so) past, though I can't remember the occasion, and was told it couldn't be done, and that the agent had to rebook my entire reservation.
They can still do it if they have no other choice and there is a good reason.
Case in point: Last year I was traveling from La Crosse, WI to New York City. My tickets looked like this:
LSE to CHI (1 of 1) Empire Builder (Coach)
CHI to PGH (Pittsburgh) (1 of 2) (Coach)
PGH to NYP Pennsylvanian (2 of 2) (Business Class)
I had used a voucher as payment and had to have the tickets printed in advance.
Then, the sleeper price from CHI to PGH fell unexpectedly to the lowest bucket so I called over the phone to grab it.
Naturally, I explained the situation that I already had printed tickets for coach and needed an accomodation only ticket.
The agent said he was unable to and told me to exchange my coach tickets at the station toward the value of the new reservation. However, since I was still a couple months away from taking the trip, I wanted to secure the reservation with a credit card payment and told him I didn't waant to have to pay the rail fair again since I already had a printed ticket for the rail fair.
Meanwhile, the price on the Pennsylvanian had gone up and Business Class was now sold out. I told him that if I exchanged my CHI to PGH ticket, I would also need to exchange PGH to NYP and be forced to pay a higher fair and lose my Business Class seating.
He put me on hold and was able to make a reservation for the Roomette only from CHI to PGH.
I actually did it another time too. If you explain to them that you already have a printed ticket, offer to give them the reservation #. Once they verify the validity, they can cross reference your accomodation only ticket with your old coach reservation and it's all good.
As a side effect, it also allows you to earn more AGR points:
I am fabricating the dollar amounts but here is how it worked with my above situation for CHI to NYC.
Coach from CHI to PGH was $42 and my sleeper accomodation was $80. If it was printed as a single ticket, I would have $122 * 2 =244 AGR point
Because they were seperate, I earned the 100 point minimum for my coach ticket and 160 points for the accomodation. Total= 260