I did not think bitumen is still allowed, I know my dad would bring home gallons of creosote, it was a by-product from the coke ovens used to make the coke for the blast furnace, and the coke ovens had about every nasty one can think of, so much was made from the waste, moth balls for example.
But today it is all banned, can't use railway sleepers in the garden any more to make steps etc. Well not the old ones, new ones are made for recycled plastic. There was a move to concrete for a time.
But in real terms we can't DIY the treatment anyway, they are put in pressure vessels to be treated so the treatment is sucked into the wood far deeper than any brush on treatment will reach.
It seems as soon as some good product is produced, it is banned. I hear all the hype about plastic, which means it will bend, and is nothing to do with what it is made of, but in the main it is made from wood, so a plastic sleeper is a super treated wood sleeper!