Not sure why, but the original message was filled with "+" between each word...
Reformatted for readability:
Often, a train being autocoupled upon collision (e.g during shunting with set_autocouple() set on both moblie train and stationary wagons) will revert to manual control instead of whatever ATC command is set in the train or the wagons.
I have often vacated the area that the shunting is being done, only to come back later to find the train stopped where it would have coupled.
This doesn't seem to happen while a player loads the area though. (May be the observer effect...)