  This and companion papers by Harrington et al. and Blecic et al. present the
Bayesian Atmospheric Radiative Transfer (BART) code, an open-source,
open-development package to characterize extrasolar-planet atmospheres. BART
combines a thermochemical equilibrium abundances (TEA), a radiative-transfer
(Transit), and a Bayesian statistical (MC3) module to constrain atmospheric
temperatures and molecular abundances for given spectroscopic observations.
Here, we describe the Transit radiative-transfer package, an efficient
line-by-line radiative-transfer C code for one-dimensional atmospheres,
developed by P. Rojo and further modified by the UCF exoplanet group. This code
produces transmission and hemisphere-integrated emission spectra. Transit
handles line-by-line opacities from HITRAN, Partridge \& Schwenke ({\water}),
Schwenke (TiO), and Plez (VO); and collision-induced absorption from Borysow,
HITRAN, and ExoMol. Transit emission-spectra models agree with models from C.
Morley (priv. comm.) within a few percent. We applied BART to the {\Spitzer}
and {\Hubble} transit observations of the Neptune-sized planet HAT-P-11b. Our
results generally agree with those from previous studies, constraining the
{\water} abundance and finding an atmosphere enhanced in heavy elements.
Different conclusions start to emerge when we make different assumptions from
other studies. The BART source code and documentation are available at
https://github.com/exosports/BART.
