The McGrath Highway is in awful shape. The overpass that soars over Washington Street in East Somerville is rusting and crumbling. Work crews have rigged nets to keep falling chunks of concrete from raining down on passing motorists and pedestrians. Engineers have classified the overpass as functionally obsolete and structurally deficient, meaning the roadway either needs to be fixed, or it needs to be torn down. The bizarre thing is, the state is putting some serious effort, and money, into doing both.