WHAT ON EARTH IS THIS? UNUSUAL "WAGON" AT THE GREAT CENTRAL RAILWAY 19.11.23

It's a working replica of a piece of rolling stock required to provide early diesel locomotives with additional bring power when hauling unfitted or partially fitted freight trains (trains formed from wagons not fitted with automatic brakes).
The old steam locomotives were considerably heavier than the diesels that replaced them and therefore had a better braking ability.
The tender took the form of a hollow box, low enough to avoid obscuring the driver's view when pushed. The body was carried on a pair of former carriage bogies, which provided the automatic brakes. The body was filled with scrap steel to raise the weight of the vehicle to 35½-37½ tons; consequently increasing the available brake force. Four lamp brackets were provided at each end to display locomotive headcode discs describing the class of train - when propelled, the tender obscured the front of the locomotive, and hence the headcode.
Introduced around 1964-65, they were taken out of use when the practice of using unfitted trains was discontinued.
None survived in preservation but an operational replica was constructed on the Great Central Railway from the remnants of a Mk1 corridor coach and has been given the next number in the brake tender sequence; B964122.
Here we see the tender with EE Type 3 No. D6700 on Sunday 19th November 2023 at the Great Central Railway.

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BRAKE TRUCK

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Tender

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BRAKE @@terrier_productions

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Brake Tender@@wooltman