We've noticed small flying insects entering and leaving the gaps between our retaining railway sleepers. They're like very small wasps, but with narrower banding so they appear to have more bands than the common wasp. They're also in quite small numbers, with only up to half a dozen or so visible at any one time. And they don't appear to be aggressive. I've tried photographing them but its difficult to focus on them they're so small. My wife says they're wasps and that she reckons they've built a nest in the sleepers (?). I'm not so sure though.