GETTING TO THE BOTTOM OF IT Underground concrete mystery puzzles rail crews By RAD SALLEE Copyright 2003 Houston Chronicle Metrorail engineer Jim Schroeder had often wondered why maps of underground utilities in downtown Houston always showed the rats' nest of pipes and cables beneath Main Street detouring around the McKinney intersection. Now he knows. On Jan. 2 a backhoe operator, ripping up pavement on Main for Metropolitan Transit Authority's light rail tracks, struck concrete -- a lot of it. After clearing away about a foot of black dirt, construction workers exposed a flat block more than 6 feet square, with...