This camera battery charger circuit is designed for DCC systems and the battery is essentially used as a capacitor as it is not allowed to become discharged.
The battery also controls the voltage to the camera as any current passed through R1 that is not needed by the camera is shunted through the battery. This is an inefficient but cheap way to control the voltage. (Circuits Without Pages Of Their Own)
Source: Train Mounted Camera Battery Charger
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