Urban railroading is dirty and no-nonsense, part of the overlooked infrastructure that keeps a city running. The Zakim Bridge may be the prettiest part of Boston's transportation system, but don't count out what's in the shadow of its northern approach. Thirty-six years out of LaGrange, F40PH #1000 can still hold her own as she roars out of town, bound for Haverhill with train 215.