I’m not sure why the link didn’t show up (at least it didn’t for me), but here it is again. If that still doesn’t work, here it is in a less pleasing form:
Car one: M, N
Car two: P, R/S
car three: Q, R/S
car four: O, T
I read the question and noticed that the specific cars/order didn’t matter–just who accompanied one another. I drew the four two-passenger cars and labeled them one through four. First, I put M and N in the same car. Then, I noted that O and R were to go to separate cars (without placing them yet), For the third rule, I separated out the P and Q into different cars and noted that S could go with either. Then, I put O in the fourth car. Since R couldn’t go with O, I put T with O. I also noted that R could go with either P or Q.
I’ve checked this myself and couldn’t find anything wrong with it. Like I said, I would just like to confirm if it’s correct, and if not, learn why it isn’t.
Thanks for your time!