  The continued success of the jet substructure program will require widespread
use of tracking information to enable increasingly precise measurements of a
broader class of observables. The recent reformulation of jet substructure in
terms of energy correlators has simplified the incorporation of universal
non-perturbative matrix elements, so called "track functions", in jet
substructure calculations. These advances make it timely to understand how
these universal non-perturbative functions can be extracted from hadron
collider data, which is complicated by the use jet algorithms. In this paper we
introduce a new class of jet functions, which we call (semi-inclusive) track
jet functions, which describe measurements of the track energy fraction in
identified jets. These track jet functions can be matched onto the universal
track functions, with perturbatively calculable matching coefficients that
incorporate the jet algorithm dependence. We perform this matching, and present
phenomenological results for the charged energy fraction in jets at the LHC and
EIC/HERA at collinear next-to-leading logarithmic accuracy. We show that higher
moments of the charged energy fraction directly exhibit non-linear Lorentzian
renormalization group flows, allowing the study of these flows with collider
data. Our factorization theorem enables the extraction of universal track
functions from jet measurements, opening the door to their use for a precision
jet substructure program.
