This happened when created link between locomotive (in this case, steam) and the cart. When start running, it was broken.
Gif: https://gfycat.com/SaltyScentedAfricangroundhornbill

Can you reproduce that in a new world and/or with just railcraft?

It produces for both new world and new cart.
To re-produce:

1. Give your own steam and minecart.
2. Make a link between cart and steam loco.
3. Add fuel + water, start running slowly.
4. After start running for a while, Its lost their link.
   (Apply for electric and perpetuum.)
   Update: Its happened when using train dispenser to make a train.
   To reproduce:
5. Give your own locomotive, train dispenser, and some cart.
6. Place train dispenser, create a pattern for train and put cart here.
7. Use redstone to active, then create link for loco and 1st cart.
8. Start run, and its lost link.

I tested and cannot reproduce this. Did you get this result on a fairly laggy computer @ccgfok? As I see from the gif, your first link failed, probably because of lag.

I think I've actually seen this in my own testing. I forget if I fixed it or not. Seems like I might have.

@liach This was happen if the dynamic light for locomotive was activated and use the recomended Forge version (12.18.2.2107+). The latest Forge vesion (1.10.2-12.18.3.2254) disabled the dynamic light.
And yeah, my computer was only 2GB RAM, and the gif link I was tried to re-produce this bug.

Going back to #882... I think it might be caused by the storage in the map of cart links, or something similar.

We can only close this because this is a very rare single case and no one else reported this.