  The international PEN collaboration aims to obtain the branching ratio for
the pion electronic decay $\pi^+ \to e^+\nu_e(\gamma)$, aka $\pi_{e2}$, to a
relative precision of $5\times 10^{-4}$ or better. The PEN apparatus comprises
a number of detection systems, all contributing vital information to the PEN
event reconstruction. This paper discusses the design, performance, and Monte
Carlo simulation of the mini time projection chamber (mTPC) used for pion,
muon, and positron beam particle tracking. We also review the use of the
extracted trajectory coordinates in the analysis, in particular in constructing
observables critical for discriminating background processes, and in maximizing
the fiducial volume of the target in which decay event vertices can be accepted
for branching ratio extraction without introducing bias.
