Thanks for the file, Sean Cusson. The Drop Box “request a file” worked nice, thanks for testing my suggestion.
My observation, (and after each test, I have reset the scene to the state I got it).
First test, reverse the sequence, works nice, and the Rail needs to be moved below the spline, to have the clone orientation the same.
The clones populates the spline at frame 0 as the original file, but now move to the left instead of the right. The spline is empty at frame 500.
Second test, the original scene, I go to frame 900, set a dash (minus) to the offset value of 180 and press the dot, so it is set (turns from orange-yellow to red.)
The clones populate the spline at frame 0 as before, but move then to the left and the belt is empty at frame 500.
To your question in post #4: What I notice in all three versions, the clones stay parallel to each other, they do not turn along the belt, is that wanted? If not, perhaps set the Cloner back (rot/scale) and do it with a Plain Effector set after the Spline. (See image). The simple answer is that they follow the spline, but the axis rotate based on the settings in the Cloner object, and so they seems to be off.
I would normally do that in the Transform part, but I do not have the rest of the file so, I do it with a Plain Effector here. Note the difference in the Cloner Object: Coord. vs. Transform!
If your target is different than the outcome of my test 1+2, please let me know and I will have a look into it. I was not certain if you like the file here attached, if you like, I will do.
My best wishes