So this one is probably a minor issue and I am not 100% sure its factorio related, but I noticed these issues before the DLC so it might have been a bug even before the dlc.

Basically I have another game I played, Brotato, it was in a beta stage for its DLC, and after quitting that game and ...

If you manually drive a train at full speed, and then click the button to start automatic travel with selecting a station to drive to, the train will lose most of its speed and accelerate again instead of continuing with the speed already achieved with manual driving.

Heres a link to a replay of ...