All locosound, PS2, and PS3 engines have and require tachometer feedback to both control the speed and provide the timing signal for chuffing sounds on steam engines.
It's could be a failed tether wiring problem, a failed optical tach sensor, tape or stripes missing on the flywheel, or just simple misalignment or spacing of the sensor compared to the stripes.
I recently had to repair an engine and it was the tether for this exact problem. The user was running the engine on a tight 031 track and eventually enough hard flexing of the cable and it failed.
Again, bench testing this engine on rollers or on its side on the bench, even in conventional, the tachometer is used for chuffing and clearly in your video you had none- combine with ramping up speed runaway.
