@rl This railway is undergoing a heavy modification. This summer the whole railway will be converted to 15 kV ~ 16,7 Hz Electrification with central overhead catenary (no more separated catenaries between Sihltalbahn and Uetlibergbahn). The Be520 trainsets were withdrawn on April 18th, 2022 (The day after this picture) because of this reason. You can find more information here , here and a in-depth article here Information about the closure dates are here in English
You can find more information here , here and a in-depth article here
Information about the closure dates are here in English