Fixing conquered/flipped cities or filched tiles. Forting strategic and desert tiles. Railroading the extent of my empire. Altering my own improvements where necessary (e.g., changing farms to workshops when I get Sushi spread to a production city). For several turns before I finish researchign railroad, my workers drop what they're doing and line up, three to a tile, on the hills of my production cities and along crucial routes. That way my first turn with railroads in hand provides quite a bang.