In the Victorian Mines (and in a lot of places in the world today, I'm expecting), it is not exactly considered weird to work naked--temperatures can go as high as 100F and humidity is often pegged at 100%. Men working at Snowdown Colliery drank more than 14 litres of water in a 8-hour period. If you tried working in clothes, they would quickly get soaked and the humidity means that your would literally be swimming in your own sweat for the entire shift.

A lot of the accounts of naked women and children miners came from the Royal Commission of Inquiry into Children's Employment (1842), which interviewed many child workers on their experiences down the mines:

I hurry [i.e. dragging coal carts] 11 a-day; I wear a belt and chain at the workings, to get the corves out; the getters that I work for are naked except their caps; they pull off all their clothes; I see them at work when I go up; sometimes they beat me, if I am not quick enough, with their hands; they strike me upon my back; the boys take liberties with me sometimes they pull me about; I am the only girl in the pit; there are about 20 boys and 15 men; all the men are naked; I would rather work in mill than in coal-pit.

--No. 26. — Patience Kershaw, aged 17, May 15. (the Comissioners commented about Patience: This girl is an ignorant, filthy, ragged, and deplorable-looking object, and such an one as the uncivilized natives of the prairies would be shocked to look upon.)

A lot of the female workers tried to cover up as much as their heat tolerance and modesty would allow them, but sometimes the nature of the work makes it tricky:

“The chain [used to attach coal carts to the waists of the pullers] passing high up between the legs of these girls, had worn large holes in their trousers, and any sight more disgustingly indecent or revolting can scarcely be imagined than these girls at work. No brothel can beat it.

--Jelinger Symons, the Report of the Children's Employment Commission, appendix to the first report (1840)