By-laws in the UK are delegated legislation (see https://en.wikipedia.org/wiki/Byelaws_in_the_United_Kingdom). Parliament makes a law which grants some other body the power to make byelaws (which then come into force subject to approval by a relevant government minister). In this case the British Waterways Board used powers granted it under the British Transport Commission Act, 1954 to make canal bye-laws: http://www.britishwaterways.co.uk/media/documents/foi/legal/...

The BWB has subsequently been abolished with its powers (including the power to make and enforce byelaws) transferred to the Canal and Waterways Trust.

Railway bye-laws operate similarly.