Upon reloading or re-entering chunks with moving minecarts, they will sometimes lose momentum if on a flat rail.
I have witnessed minecarts being stationary on both normal and powered rails when they were moving before I left the chunks.
Could not find duplicate of this bug, but feel free to mark as so if there is one
(Note to moderators)
This is not a duplicate of MC-2878;
This issue refers to minecarts getting stuck and remaining stuck when reloaded, while that report is describing minecarts not being able to travel through unloaded chunks but makes no mention of them getting stuck when the chunks are reloaded.