The common nickname for the first logo of the nationalised British Railways, adopted in 1948 shortly after nationalisation and used until 1956. The logo, as I attempt to depict in the ASCII art below, consists of a spoked wheel with a bar across it with the words 'BRITISH RAILWAYS' on it, and a (rather heraldic-style) lion riding atop the whole thing, feet on either end of the bar. It doesn't look like a very comfortable or stable ride for the lion, either, and it was also nicknamed the Lion on a Unicycle.
The spoked wheel does look rather like a dartboard, too. As to the ferret part, I have not found any explanation that makes sense.
The term originally carried a somewhat derogatory meaning, largely thanks to the unpopularity of the nationalisation among some employees of the absorbed railway companies. With the fading of such feelings in time, and the fact that British Railways/British Rail itself is no more, the usage generally lacks such a meaning in the modern day.
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