  We present ~120,000 Spitzer/IRAC candidate young stellar objects (YSOs) based
on surveys of the Galactic midplane between l~255 deg and 110 deg, including
the GLIMPSE I, II, and 3D, Vela-Carina, Cygnus X, and SMOG surveys (613 square
degrees), augmented by near-infrared catalogs. We employed a classification
scheme that uses the flexibility of a tailored statistical learning method and
curated YSO datasets to take full advantage of IRAC's spatial resolution and
sensitivity in the mid-infrared ~3-9 micron range. Multi-wavelength
color/magnitude distributions provide intuition about how the classifier
separates YSOs from other red IRAC sources and validate that the sample is
consistent with expectations for disk/envelope-bearing pre-main-sequence stars.
We also identify areas of IRAC color space associated with objects with strong
silicate absorption or polycyclic aromatic hydrocarbon emission. Spatial
distributions and variability properties help corroborate the youthful nature
of our sample. Most of the candidates are in regions with mid-IR nebulosity,
associated with star-forming clouds, but others appear distributed in the
field. Using Gaia DR2 distance estimates, we find groups of YSO candidates
associated with the Local Arm, the Sagittarius-Carina Arm, and the
Scutum-Centaurus Arm. Candidate YSOs visible to the Zwicky Transient Facility
tend to exhibit higher variability amplitudes than randomly selected field
stars of the same magnitude, with many high-amplitude variables having
light-curve morphologies characteristic of YSOs. Given that no current or
planned instruments will significantly exceed IRAC's spatial resolution while
possessing its wide-area mapping capabilities, Spitzer-based catalogs such as
ours will remain the main resources for mid-infrared YSOs in the Galactic
midplane for the near future.
