I recently purchased the subject set for my grand daughter and we both found it delightful though I noted some issues that really come down to quality control problems. In assembling the fast track some of the track did not simply snap together but in fact required quite a bit of force to join them. Fast track is relatively expensive but does not appear to be engineered as well as it should.
In running the set around the oval the coupler of one of the cars repeatedly opened separating them. I checked the manual and this appears to be a known problem. The recommendation was to use needle nose pliers to bend a pin. This set is not particularly cheap and it should not be necessary to bend pins to keep cars together.
The remote control aspect did work quite well and was easy enough for my 3 and 1/2 year old grand daughter to operate the train to our mutually delight. She was almost beside herself in her joy making what I spent worth every dollar.
JohnF
You are right that they should leave the factory such that the couplers will both stay closed if not intentionally set off by the mechanism and also should uncouple when intended (like most of the cars produced have for probably at least 60-70 years, with a rare defect exception here or there).