Hmm!
Many the cast iron bath I have lifted upstairs by myself. Its a tortoise job and if you have an apprentice a finger in the overflow hole. Not taken many down stairs by myself though, they run away with you. Seen them go down the stairs like a sledge and crash into whatever is at the bottom.
And anyway the apprentices wised up, and you could then have a fight on your hands, for who got at the back and who went down the stairs in front of the bath and stood a better chance of getting squashed or run over.
The thing is, if you started to laugh at any stage it was murder, especially on a turn or awkward bit, you usually could not stop laughing and the bath seemed to get heavier and heavier which made you laugh even more, laughing out of fright possibly.
Try breaking it up with a lump hammer? Your head is like a having a set of bells installed for about a week afterwards.
Being honest they weren't half heavy, I was made up when they started using pressed steel and plastic.
One thing though, you did not have to worry about them moving. But if you did not get in them quickly, after you filled it with hot water, they soon went cold.