Found this lovely photo today. Paddington Goods 1926. Looks as if the horses in the foreground are being carefully manouvered to avoid the weighbridge. Note the absence of a rail alongside the weighbridge. It does not seem to have been a requirement, especially in the pre-grouping years.
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I have been browsing photos to see if I could find any evidence of the GWR's own horse-drawn vehicles ever being weighed. No luck yet. I suppose it would mostly have been non-GWR vehicles, although a large single shipment carried on a GWR carriage could have been weighed this way (if you see what I mean).
Edit: Although having said that, this companion photo just turned up. But is that a GWR wagon? And is it being weighed? It was taken during the General Strike, so this is not necessairly business as usual.
Source: Getty images. Embedding permitted.