[Posted by Prof Henry G. Overman] Last November, at an event in Leeds with the Deputy Prime Minister, I expressed some scepticism that transport investment across the North would create a Northern Power House. I argued that two pieces of evidence about city success urge greater caution: 1) The evidence on agglomeration economies - i.e. on the factors that underpin the success of cities - suggest that these decay quite strongly with distance. So a key part of London's success, for example, comes from the very high employment densities in central London. 2) A crucial driver of city success is the concentration of high skilled workers that are found in our most successful cities