Abstract. Wind energy has witnessed a staggering development race, resulting in higher torque density demands for the drivetrain in general and the gearbox in particular. Accurate knowledge of the input torque and suitable models are essential to ensure reliability, but neither of them are currently available in commercial wind turbines. The present study explores how a subspace identification framework, using distributed fiber-optic strain sensors on a four-stage gearbox, can provide input torque measurements through the use of operational deflection shapes. Compared to conventional gear tooth root strain gauge measurements, an innovative measurement setup with 129 fiber-optic strain sensors has been installed on the outer surface of the ring gears to research the deformations caused by the gear mesh events. Consistent estimates of the deflection shapes have been found by applying the Multivariable Output-Error State-sPace (MOESP) subspace identification method to strain signals measured on a serial production end-of-line test bench. These operational deflection shapes, driven by periodic excitations, account for almost all the energy in the measured strain signals. Their contribution is controlled by the torque applied to the gearbox. From this contribution, a torque estimate has been derived for dynamic operating conditions. Accurate knowledge of the input torque throughout the entire service life allows future improvements in assessing the remaining useful life of wind turbine gearboxes. Additionally, tracking the operational deflection shapes over time is proposed to enhance condition monitoring in planetary gear stages.