Said in an equally cheerful way, the Highland Railway used specially made ramps for their double deck sheep wagons. You're quite right, loading livestock would have been a pain and the wagons would have been used for all types of load. Then again if it's all you've got and there is a demand to move livestock that's what you'll use. As you'll know the alternative in that era was droving. I have a 1951 book, 'Wales in Maps', University of Wales Press, that tells how a cattle drove from Llyn to London would take two months so perhaps they were used for livestock until the development of a wider range of rolling stock.
Your beautiful models illustrate a time of tremendous change.