## 1 Introduction  
Electron dynamics on the atomic scale occurs on an attosecond time scale as dictated by quantum mechanics. Routinely, two-color photoionization schemes with XUV attosecond pulses obtained by high-order harmonic generation (HHG), driven by table-top laser sources, and a phase-locked near-infrared (NIR) dressing field are employed to investigate the dynamics with attosecond temporal resolution [1]. These experiments demand a subcycle synchronization between the XUV pump and the NIR dressing fields to probe the electron dynamics. The temporal characterization of the attosecond pulses used in the two-color schemes is a fundamental prerequisite for their application in pump-probe experiments. Attosecond pulse trains (APTs) and the isolated attosecond pulses generated in the HHG process are usually characterized using the reconstruction of attosecond bursts by the interference of two-photon transitions (RABBITT) and streaking techniques, respectively [2; 3]. Often, both the RABBITT and streaking approaches, initially developed as characterization schemes, are utilized for investigating different processes such as photoionization dynamics in atoms and molecules [4; 5]. However, a few limitations of the HHG process restrict their application. Apart from the very low conversion efficiency of the HHG process, the harmonics generated are hard to manipulate in phase, amplitude, and polarization, thus limiting the pulse-shaping capability [6; 7]. Free-electron lasers (FELs) are sources of radiation spanning the XUV and X-ray spectral ranges, characterized by durations ranging from a few femtoseconds to tens of femtoseconds and by unprecedented peak intensities [8; 9; 10]. The advent of FELs led to the inception of schemes to determine the structural properties of single macromolecules exploiting the high number of photons interacting with the sample [11]. The ultrashort pulse duration of the radiation bursts, on the other hand, led to several experiments investigating time-resolved studies of various molecular dynamics [12; 13; 14; 15]. However, performing two-color experiments at the FEL facilities presents the formidable challenge of actively controlling the delay stability between the XUV and the external laser fields. Presently, the delay jitter between the two colors at various FEL facilities is found to be much higher than that needed for attosecond time-resolved experiments [16; 17]. Until recently, the generation of attosecond pulses at the FEL facilities has been restricted to theoretical schemes [18; 19; 20; 21]. Only in 2020, isolated attosecond pulses were produced at the LCLS [22], and attosecond pulse train generation was observed at the FEL FERMI [23]. The methods employed for the experimental observation of the attosecond pulses at these facilities differ significantly. The attosecond pulses generated at the LCLS suffer from shot-to-shot fluctuating spectral properties, which corresponds to a varying temporal structure of the attosecond pulses. This is the result of the self-amplified spontaneous emission (SASE) process, which calls for a pulse characterization technique able to provide pulse characteristics on a single-shot basis. On the contrary, at FERMI, the seeding mechanism ensures that the spectral properties are reproducible from shot-to-shot, and a multishot technique can be implemented for the temporal characterization of the XUV radiation. Moreover, while the technique demonstrated at the LCLS closely follows the attosecond streaking approach adopted for the characterization of isolated attosecond pulses, the method demonstrated at FERMI was inspired by the RABBITT technique used in the attosecond community for the characterization of attosecond pulse trains. In this article, we present the generation of APTs and their shaping at the FEL FERMI using the beating of multiple phase-locked harmonics, which is similar to the scheme proposed to generate subfemtosecond waveforms using multiple table-top lasers [24].