oristar, what's a couchette? Is it like a sleeper car or a private car?
Thinking of Amtrak, where most seats are just that but some trains have sleeper cars, I'd just call either of them "ticket" or "train ticket" -- after all you sit when you're awake either way... I suppose I might say "sleeper ticket" but I wouldn't qualify just an ordinary train ticket by calling it a "seat ticket"... maybe tickets could be referred to by First Class or Second Class, etc.?
The couchette ticket here means " a ticket for sleeping berth/sleeping car/sleeper car".
A ticket for seat means when by train you can just have a seat with which you can only sit there -- if you want to sleep, all right, sit there and try to sleep! Don't expect you can lie down and have a comfortable sleep. Otherwise, you should buy a ticket for sleeper car. And of course, the ticket is more expensive than the ticket for a seat.
For example, if you want to go to X city from N.Y. by train, a ticket for a seat just costs $10, while a ticket for sleeper car would cost you $20! Yes, as you have described, you are quite free to sit or lie down if you've bought a ticket for sleeper car. But if you have just bought a ticket for seat, well, there is no enough roomy place for you to lie down comfortably.
PS.(1) Personally, I think calling a ticket for a seat on train "seat ticket" is acceptable, unprofessional though. But no one will get it wrong.
(2) I didn't mean or I haven't asked the question of "first class" vs "second class".