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.