  We present transverse momentum and rapidity spectra of charged pions in
central Ru + Ru collisions at 400$A$ and 1528$A$ MeV. The data exhibit enhanced
production at low transverse momenta compared to the expectations from the
thermal model that includes the decay of $\Delta(1232)$-resonances and thermal
pions. Modification of the $\Delta$-spectral function and the Coulomb
interaction are necessary to describe the detailed shape of the transverse
momentum spectra. Within the framework of the thermal model, the freeze-out
radii of pions are similar at both beam energies. The IQMD model reproduces the
shapes of the transverse momentum and rapidity spectra of pions, but the
predicted absolute yields are larger than in the measurements, especially at
lower beam energy.
