The XFEL RF system is described in section 4.2.4 of the Technical Design Report (TDR).
The RF power for the XFEL linac is generated by klystrons which are installed in the underground linac tunnel (XTL), close to the accelerator modules. The klystrons are powered by HV pulses from modulators which are located in the modulator hall (XHM), above ground on the DESY site. Each modulator is connected to the pulse transformer at the klystron by means of an up to 1.5 km long triaxial cable.
This scheme allows for accelerator maintenance without a second (service) tunnel.