Analyzing galaxy interactions in the local universe can reveal how galaxies have evolved over time. The most luminous local galaxies tend to be interacting systems that radiate most of their energy at infrared (IR) wavelengths. A fundamental, yet unresolved question is if the energetic processes seen in local IR-luminous galaxies also power distant IR-luminous galaxies in the early universe. Here we use far-infrared (FIR) spectroscopic and continuum measurements obtained with SOFIA’s Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) to constrain the physical conditions in the interstellar medium of a sample of 8 local IR-luminous galaxies. We present a detailed analysis for one particularly interesting source, the nearby LINER galaxy NGC 2623. We also pioneer new techniques to measure faint OH+ absorption lines to better understand the interstellar chemistry in all eight sample galaxies. Our new photometric data confirms existing spectral energy distribution (SED) models for NGC 2623. We also find the first evidence after Herschel PACS of enhanced cosmic ray emission in Ultra Luminous IR Galaxies (ULIRGS). We find detections of OH+ absorption lines in the spectra of seven sample galaxies, and we argue that these absorptions may be due to a greatly enhanced level of cosmic rays, perhaps from shocks of star formation, that ionize this molecule. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-1852268, and by the Smithsonian Institution.