I will describe a series of experiments with a bundle of 1D gases, consisting of ultra-cold atoms in a 2D optical lattice. We take these gases out of equilibrium by suddenly changing the axial trap potential depth by as much as a factor of 100. Generalized hydrodynamics (GHD) describes the out-of-equilibrium behavior of nearly integrable systems, like these 1D gases. GHD is a hydrodynamic theory that assumes local equilibrium, while taking into account the many contraints associated with the distribution of rapidities, which are the momenta of the quasiparticles that emerge from mutual interactions among the particles. We have developed a way to measure the evolving distribution of rapidities, making this abstract concept a physical observable. From strong to intermediate coupling, with as few as 10 and a many as 150 particles per 1D gas, and over several oscillation cycles, we find that GHD describes the experiments very accurately. We have thus validated GHD's assumptions for real experimental conditions.