The skirt, or what I prefer to call "an apron" is what is used to deter diggers. Mine is 24" wide and is laid flat on the ground. The part that abuts the structure is bent into an upright L of 4 t 6 inches, so you have something to fasten to the vertical wall of the house or run.....or in your case, perhaps the railroad ties. Fasten it in place with either fence staples or screws and washers. I used fence staples.
I used 1" x 2" 14 gauge welded wire. That size should stop all but the smallest diggers, including a mink or weasel. It would not stop rats, as they would likely tunnel in from somewhere else, or could even tunnel through 1" opening.
With an apron of this type, diggers start the base where vertical meets horizontal....and immediately hit the wire and are stopped cold. They may cast up and down the line trying new digs, but always hit the same dead end. They are not smart enough to back off a couple feet and start their digging operation way out there so as to tunnel in.
To install the apron, cut down any grass or vegetation down to nearly bare dirt, then lay apron flat on the ground and pin down the corners and edges with landscape staples. Grass will grow back through it and in a short while, you can mow right over it or weed eat along the edge to keep the grass down.
Get the apron right and it won't matter what it is doing the digging. They all meet the same dead end.