Momentum spectra of charged pions over nearly full rapidity coverage from target to beam rapidity have been extracted from 0−5% most central Au+Au collisions in the beam energy range from 2A to 8AGeV by the E895 experiment. Using a thermal parametrization to fit the transverse mass spectra, rapidity density distributions are extracted. The observed spectra are compared with predictions from the RQMD version 2.3 cascade model and also to a thermal model including longitudinal flow. The total 4π yields of the charged pions are used to infer an initial-state entropy produced in the collisions.12 MoreReceived 1 July 2003DOI:https://doi.org/10.1103/PhysRevC.68.054905©2003 American Physical Society