https://forums.factorio.com/110522

lol that's a new one 😅. There was probably some bug or weird interaction with another mod that caused the car to exist beyond its use time. I guess the best fix method would be to periodically check for cars that are no longer tied to some function and delete it

Not exactly, at the time of the bug report this bug was possible to reproduce in MP by having a player disconnect while it was traveling using zipline and i was able to reproduce it locally without any other mods. Basically the bug was that when player disconnected the RTPropCar would keep moving with constant speed not following those wires anymore.

I see so it was a disconnect in multiplayer while they were moving. by 'locally' do you mean it also happens in single player?

Single player is not exactly a way to reproduce it because Factorio may try to reuse the only player that was connected in the game. However i just tested Factorio 1.1.103 with Renai Transportation 1.2.8 only and the issue was possible to reproduce using following steps:
1/ Host new game locally as planer named "A"
2/ Build zipline contraption (2 electric poles, 2 terminals, zipline trolley, programmable controller)
3/ Start moving from one terminal to the other terminal
4/ While character is moving, quickly save the game to a zip
5/ Change your local player name
6/ Host game using save file in step 4
7/ Because game is hosted the game is not allowed to reuse existing player and because local player name is now different you will join as a second player. First player is now disconnected and the RTPropCar will keep moving.

No steps require connection locally or through the network, just hosting the game and being the only player is enough.