Let's go the the source to answer our disagreements.
https://www.ada.gov/pubs/adastatute08.htm#12162a" onclick="window.open(this.href);return false;
Sec. 12162. Intercity and commuter rail actions considered discriminatory
(a) Intercity rail transportation
(1) One car per train rule
It shall be considered discrimination for purposes of section 12132 of this title and section 794 of title 29 for a person who provides intercity rail transportation to fail to have at least one passenger car per train that is readily accessible to and usable by individuals with disabilities, including individuals who use wheelchairs, in accordance with regulations issued under section 12164 of this title, as soon as practicable, but in no event later than 5 years after July 26, 1990.
(2) New intercity cars
(A) General rule
Except as otherwise provided in this subsection with respect to individuals who use wheelchairs, it shall be considered discrimination for purposes of section 12132 of this title and section 794 of title 29 for a person to purchase or lease any new rail passenger cars for use in intercity rail transportation, and for which a solicitation is made later than 30 days after July 26, 1990, unless all such rail cars are readily accessible to and usable by individuals with disabilities, including individuals who use wheelchairs, as prescribed by the Secretary of Transportation in regulations issued under section 12164 of this title.
(B) Special rule for single-level passenger coaches for individuals who use wheelchairs
Single-level passenger coaches shall be required to
(i) be able to be entered by an individual who uses a wheelchair;
(ii) have space to park and secure a wheelchair;
(iii) have a seat to which a passenger in a wheelchair can transfer, and a space to fold and store such passenger's wheelchair; and
(iv) have a restroom usable by an individual who uses a wheelchair, only to the extent provided in paragraph (3).
So, we're both correct in a way. Trains only require one car with wheelchairs assessibility - but all new cars must built after 1990.
So, to answer our hypertherotical slumber sleeping car scenario,it depends upon whether we use old mothballed cars or brand new cars.
While the wheelchair space must be adjacent to the designated slumber bunk, a single handicapped restroom in the car near that accessible designated bunk would meet the regulations. The wheelchair space could be placed across the aisle from the restroom with the first lower bunk being designated as the wheelchair assessable seat/bunk. So it should be quite easy to make a slumber coach wheelchair assessable.