Grid-forming control is a promising solution for enhancing the performance of AC-AC modular multilevel converters (MMCs) in railway traction systems, facilitating both independent operation and multi-converter synergy. However, implementing grid-forming control for AC-AC MMCs without a DC link, which provides stable voltage and frequency support as well as damping and inertia to the system, requires further investigation. To address this need, this work proposes a grid-forming control scheme for AC-AC MMCs, which includes grid-forming control for the single-phase side, energy maintaining control for the three-phase side, and voltage balancing control. By leveraging the energy stored in the module capacitors, a synchronization structure with virtual admittance is developed for the single-phase grid-forming control, effectively emulating the dynamic behaviors of rotary frequency converters (RFCs). The energy maintaining control for the three-phase side and the voltage balancing control are subsequently derived to support the overall control scheme. Experimental results validate the effectiveness of the proposed grid-forming control scheme using a scaled-down AC-AC MMC.