The transient response of a 0.4 by 0.6 cm rectangular bore rail accelerator was analyzed using a three-dimensional finite-element code. Results are presented for the case of a 210-kA current input and 5-cm arc length. Typically, the copper rail deflected to a peak value of 0.08 mm in compression and then oscillated at an amplitude of 0.02 mm. Simultaneously the insulating side wall of glass fabric base, epoxy resin laminate (G-10) was compressed to a peak value of 0.13 mm and rebounded to a steady state in extension. Projectile pinch or blow-by due to the rail (or side-wall) extension or compression, respectively, can be identified by examining the time history of the rail (or side-wall) displacement. For the case presented, the effect of blow-by was most significant at the side wall characterized by mm-size displacement in compression. Dynamic stress calculations indicate that the G-10 supporting material behind the rail is subjected to over 21 MPa - at which the G-10 could fail if the laminate was not carefully oriented. Results for a polycarbonate resin (Lexan) side wall show much larger displacements and stresses than for G-10. Therefore the tradeoff between the transparency of Lexan and the mechanical strength of G-10 for side-wall material is obvious. Displacement calculations from the modal method are smaller than the results from the direct integration method by almost an order of nagnitude, because the high-frequency effect is neglected. However, this effect is significant and dominating for a highly-impulsive, wave-propagation problem such as the rail accelerator structure.