Measurements are shown of GeV pions and muons in two 300μm thick, Si Medipix pixel detector assemblies that are stacked on top of each other, with a 25μm thick brass foil in between. In such a radiation imaging semiconductor matrix with a large number of pixels along the particle trail, one can determine local space vectors for the particle trajectory instead of points. This improves pattern recognition and track reconstruction, especially in a crowded environment. Stacking of sensor planes is essential for resolving directional ambiguities. Signal charge sharing can be employed for measuring positions with submicron precision. In the measurements one notices accompanying 'delta' electrons that emerge outside the particle trail, far beyond the boundaries of the 55μm pixel cells. The frequency of such corrupted position measurements is ~ one per 2.5mm of traversed Si.