The Stock Exchange Nicknames of British Railway Companies - 1898

In my researches yesterday, I came across a list of the stock exchange nicknames of Britain's largest railway companies in 1898. What I found interesting is that some of the names just use a word, or part of a word, from the company's title. However, other names relate to the region the companies served, for example the North Staffordshire Railway was referred to as 'pots', presumably because of the large pottery industry there. I'm now just curious about how these names came about. The whole list was as follows:-

Caledonian                                          Claras
Cambrians                                          Cambs
Glasgow and South Western                Ayrshires
Great Central                                      Centrals
Great Eastern                                     Easterns
Great Northern                                    Yorks
Great North of Scotland                       Haddocks
Great Western                                    Westerns
Hull and Barnsley                                Hulls
London and North Western                  Brums
London and South Western                 Souths
London, Brighton and South Coast       Berthas
London, Chatham and Dover                Little Chats
Midland                                              Middies
North British                                       British
North Eastern                                     Berwick
South Eastern                                    Doras
Lancashire and Yorkshire                    Leeds
North Staffordshire                             Pots
Taff Vale                                            Taffs


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[1] The National Archives, ZPER 46/1, The Railway Yearbook for 1898