[en] We compare transition radiation (TR) and ionization energy loss distributions obtained from a Monte Carlo simulation based on Geant4 to electron and proton data. The data were taken with a prototype consisting of 20 layers of fleece radiator and straw tube proportional chambers, filled with Xe/CO₂, during a testbeam at CERN. We find impressive agreement for the simulation of TR for electrons and very good agreement for the ionization loss distributions for protons over a wide range of incident energies. The implications of slight deviations in the tails of the proton energy loss spectra on calculated proton suppression factors are studied briefly