Animals by contrast can be kept out by suitable fencing, lack of suitable or properly maintained fencing is the responsibility of network rail, and therefore lack of same is the fault of the industry.
The railways have to keep a fence that is proof to livestock. Deerproof fencing is a whole other level. After having collided at high speed with a fairly small deer (a muntjac) on the A34 Newbury by-pass a few years back, which wrote-off my car, I was very pleased to see that the Highways Agency installed deerproof fencing along the stretch. Mine had been one of several collisions that week, according to the recovery man. A deerproof fence is much higher, and much more sturdily constructed than conventional livestock fencing. Most deer species have little trouble in leaping over livestock fencing.