It is an interesting thought! Presumably it was considered than your average train can withstand six or so, but not double figures! Who knows!
Regarding swans, it is surely high time that the frankly silly business of stopping mainline trains because one wanders into the fourfoot was scrapped, 'royal' birds or not?! Bl**dy things. The current rule seems to have been written as best it can be to keep within the legal technicalities whilst, perhaps, offering a little room for interpretation, in that trains only need be cautioned if the thing is actually in the fourfoot (nudge, wink...), but really it needs scrapping. 'Tis fine to pulp a small flock of sheep at full tilt but not a solitary swan.