Hanging from the sides or rear of lorries was quite the thing in those days, even in my day [!].  I well remember doing what Charlie used to do, very naughty, but very young mannish.  Goods vehicles were very underpowered compared with now, and they did use to struggle going uphill.

But analysing the terrain this day, severe Pennine hills, I suppose they could be forgiven, and they were in unknown territory.  Carlines he mentions, are better known now as tramlines.