I agree with everything RTT_Rules has said, but I feel compelled to point out that the all-"business" Cairns Tilt Train is already a little luxurious. If we really wanted to serve the proles with cattle class, we'd need a proper economy cabin too - perhaps the Rockhampton Tilt Train?
I'm sure it's possible to install roomette accommodation as a sort of sleeping car - if you were to use double-bunk roomettes with a zig-zag corridor, you could probably fit around 36 berths in a car. (Or spread a similar number over two cars with other facilities, such as a lounge, dining area and/or showers/bathrooms). Whether or not you can fill them all with paying passengers is a different story.
I think you are right, the CTT seating is too spacious for the whole train. But perhaps it gets down to the technology of the track, being skinny gauage and skinny above rail profile. The RTT may rule as a train, but EC seating on that train absolutely sucks. The spacing is too tight in leg room and the narrow width doesn't help comfort for 5-6hr trips I have used it. Actually puts me off using the train. BC yes, like the airlines there was intially a big gap between the two in price and service. Removal of the silver service brought the cost down to make it more viable.
I'd dearly like to see the EC seats on the RTT improved, perhaps they have since the refit, I have not used it since. Ok I'm tall, but it needs another few inches of leg room, so remove 1-2 rows of seats and then maybe you could make the recline a bit more. The narrow car bodies limit the width. The same would then be suitable for the CTT and fit the car with only overhead TV's like RTT, but I think only the desperate would travel the full distance, but would be great for shorter haul at lower cost.
I know its all relative and many smeg about the XPT and while I have only used the sitters a few times, really for a tall person I thought it was great. We were in car 7 and travelled in the 90's, so fairly sure it was EC. Since then I've only done sleeper because it was cheap at the time.
I don't know how many twin berths you'd get, I doubt you'd get 36 people. the XPT sleeper only has 18 beds, getting rid of the 5 dunnies/showers shared between two cabins with a dunny at each end + 1 shower at one might save some space. The XPT users the older style 4 exits doors per car where as the CTT cars only have twin doors which saves a bit of space.
But it just goes to show, in the XPT there are 18 beds in a sleeper, EC has 68 seats, hence the ticket price needs to be nearly 4 x higher for the railway to cover the cost. For a FF pax, you really need to love train travel to pay that over EC for a 1500km trip instead of flying. XPT FC is just over 1.2 x the price and hence not much to upgrade for more legroom and better seats.