I just checked that. Started the scenario, pressed Escape and Save. Then quit to Menu. I can resume it like any other scenario.
I have tried. Started scenario, uncoupled and stopped at Mountain Lake Park Platform 1. Saved the game. Quit to main menu. Selected the scenario and hit the "Resume" button. I could continue without problems and the game had been saved correctly. I suppose you were clicking the "Replay" button instead after quitting the scenario. This always restarts the scenario from the start. You have to click on Career tab and select the scenario which takes you to the "Resume" button.
I've done that one recently I'm sure it worked properly for me as I seldom have time to complete a scenario in one sitting. Did trailer jet pt 4 work for you I couldn't get it to acknowledge stopping at one point? I'll replay soon. Enjoy the drive.
I've completed that one successfully. And I remember a discussion on the Steam forums where some players claimed there was an error in the scenario, which is not true. The most likely task to fail an uncoupling instruction due to being on a downward grade, leading to the cars to recouple which is an Operational error. I'm quoting my post from the Steam forum: