When you hop on the rail, you force the character to follow along, disregarding the character movement comp:
Here, right? I feel you might be making the character movement comp angry. Could you try deactivating it for the duration of the spline ride?
Were I to attempt something along the lines of:
The first thing I’d do, would be to get the char movement comp out of the way as it tries really hard to do its thing, whatever it is in your case - but that does look wonky, indeed.