This is most easily accomplished on level 4-5; you're going to have to destroy some buildings anyway as part of your building railroad tracks, and you can get large amounts of money on this map. Try to max out your money and minimize track-building, then as your last train is pulling into the station, demo buildings (note: buildings! Trees don't count! I'm not sure if construction sites do, either - to be safe, demolish houses and skyscrapers) until you're almost out of money, then complete the map.