We investigate superconducting transmission lines as a novel platform for realizing a quantum fluid of microwave photons in a propagating geometry. We anticipate that the strong photon-photon interactions provided by the intrinsic nonlinearity of Josephson junctions are sufficient to enter a regime of strongly interacting photons for realistic parameters. To prove the potential of this platform, we show that a suitable tapering of the transmission line parameters allows for the adiabatic conversion of an incident coherent field into a cold Tonks-Girardeau gas of fermionized photons close to its ground state. Signatures of the strong correlations are visible in the correlation properties of the transmitted light.