  With the increase in energy of the Large Hadron Collider to a centre-of-mass
energy of 13 TeV for Run 2, events with dense environments, such as in the
cores of high-energy jets, became a focus for new physics searches as well as
measurements of the Standard Model. These environments are characterized by
charged-particle separations of the order of the tracking detectors sensor
granularity. Basic track quantities are compared between 3.2 fb$^{-1}$ of data
collected by the ATLAS experiment and simulation of proton-proton collisions
producing high-transverse-momentum jets at a centre-of-mass energy of 13 TeV.
The impact of charged-particle separations and multiplicities on the track
reconstruction performance is discussed. The efficiency in the cores of jets
with transverse momenta between 200 GeV and 1600 GeV is quantified using a
novel, data-driven, method. The method uses the energy loss, dE/dx, to identify
pixel clusters originating from two charged particles. Of the charged particles
creating these clusters, the measured fraction that fail to be reconstructed is
$0.061 \pm 0.006 \textrm{(stat.)} \pm 0.014 \textrm{(syst.)}$ and $0.093 \pm
0.017 \textrm{(stat.)}\pm 0.021 \textrm{(syst.)}$ for jet transverse momenta of
200-400 GeV and 1400-1600 GeV, respectively.
