The tracker geometry and materials description is found in the titles file //titl directory tgeom13 (TP version). The source code which interprets these titles and makes the GEANT detector description is in //detc file and the tracker related code is in the directory //detc/dtgeom. For more details please consult the chapter 'Detector description ...'.
There are title examples for materials description in Appendices A.2-A.3. Some geometry titles examples with figures can be found in Appendix A.5.
The detailed tracker geometry which comes by default (or by GEOM 'TRAK' 3) contains detector description including supports, cables cooling pipes and electronics chips. The other geometry description options are average (GEOM 'TRAK' 1) and simple (GEOM 'TRAK' 2).
In simple geometry option the tracker is described as simple cylinders of thickness 0.3 mm and 2 mm for Si and MSGC, respectively. No electronics, support structures nor cooling pipes are described. The material density is adjusted so that the total thickness in terms of radiation length corresponds to the average in design.