Understanding relaxation processes is an important unsolved problem in many areas of physics. A key challenge in studying such non-equilibrium dynamics is the scarcity of experimental tools for characterizing their complex transient states. We employ matter-wave interference to study the relaxation dynamics of a coherently split one-dimensional Bose gas and obtain comprehensive information about the dynamical states of the system [1, 2]. Following an initial rapid evolution, we observe the approach towards a thermal-like steady state characterized by an effective temperature that is independent from the initial equilibrium temperature of the system before the splitting process. We conjecture that this state can be described through a generalized Gibbs ensemble and associate it with pre-thermalization. New measurements further elucidating the nature of the steady state will be presented.