We introduce a circuit quantum electrodynamical setup for a quantum single photon transistor. In our approach single photons propagate in two open transmission lines that are coupled via two interacting transmon qubits. The interaction is such that photons are not exchanged between the two transmission lines but a photon in one line can completely block respectively enable the propagation of photons in the other line. High on-off ratios can be achieved for feasible experimental parameters. Our approach is inherently scalable as all photon pulses can have the same pulse shape and carrier frequency such that output signals of one transistor can be input signals for a consecutive transistor.