Verdigris (3rd Doctor)

Iris Wildthyme and her new friend Tom visit the Doctor at home in 1973. While out on a day trip, a single carriage materializes out of nowhere, full of comatose bodies in eighteenth century dress. As they arrange for these people to be taken to the hospital, they are watched by an oddly-dressed boy and gift, who quickly vanish. Meanwhile the train passengers die and crumble into a green powder -- Verdigris.