The car nearest to us in this photo is protected with what are known as "blue flags" (although they're actually metal). Such cars are not supposed to be moved until the flags are taken down by whoever put them up. In addition, the wheel on the right is chocked, and the chock is meant to be visible. Besides this, there is a derail on this track out of view to the left so that if the car were to somehow come loose, it would deliberately derail rather than rolling free onto the other house tracks. On top of all that, the rails themselves between the derail and the nearest of the two house tracks is completely filled with earth. It looks like whenever crews put cars into this siding, they do it exclusively from the opposite (east) end.
While walking up to take the previous photo, Lisa spotted this: a broken rail and a broken rail joint. The broken rail looks to me like they put the rail joint around it to hold it together, but the joint has broken as well. While they generally move cars pretty slowly, it does seem to me like it should be a cause for concern. The track with the broken rail is one of the two running rails for "old house track" (the one farthest from the main line), while the less-shiny rail in the foreground is one of the two rails that runs into the Imerys siding.
If I see one of the maintainers, I'll point them at what we saw in the yard and ask if they need to do something about it. I certainly don't want anything derailing in our yard, even at low speed, considering what some of those cars contain.