Hello, I can answer this. When lifespan runs out on your wagon, it'll start moving at around 75% of its original speed. Now I'm not 100% sure on this since I was afk but it might also start losing HP since the lifepoints are already at 0. I'm on my 5th wagon now and unless I'm close to a stable I just sell my wagon at around 5k lifepoints.
Just FYI, you lose 3 lifepoints per second of travel , at least on the 180k farm wagon. Hope this helped.