>coals to Newcastle - A shortened form of "That's like taking
>coals to Newcastle." Newcastle-upon-Tyne was a famous point of export
>for English coal, so the idea of taking coals to Newcastle is a
>shorthand for taking the trouble to do something unnecessary.

Notably, there was a businessman named Timothy Dexter who shipped coal to Newcastle after being convinced it was a common expression due to its good sense.

They arrived in the middle of a mining strike.