did not appear on the Tube map since the Stringemore pocket map of 1926. The latest removal resulted in widespread international media attention, and general disapproval from most Londoners as well as from the then Mayor of London, Boris Johnson. Based on the reaction, the following edition of the diagram in December 2009 reinstated both the river and fare zones. In more recent years, TfL has expanded its rail services, notably with the expansion of the London Overground network, which has taken over a number of National Rail lines and brought them into the TfL network, each of them being converted lines being added to the Tube map. Further additions have been made such as the London Cable Car and the boundaries of fare zones. Some commentators have suggested that Beck's design should be replaced with a new design that can incorporate the new lines more comfortably. Pocket map covers Since 2004, Art on the Underground has commissioned various British and international artists to create a cover for the pocket map. These free maps are one of the largest public art commissions in the UK, with millions of copies printed. Over 35 different designs have been produced, from artists such as Rachel Whiteread, Yayoi Kusama, Tracey Emin and Daniel Buren. Technical aspects The designers of the map have tackled a variety of problems in showing information as clearly as possible and have sometimes adopted different solutions. The font for the map, including station names, is Johnston, which uses perfect circles for the letter 'O'. That is historic and the generic font for all TfL uses from station facades to bus destination blinds. Line colours The table below shows the changing use of colours since Beck's first map. The current colours are taken from Transport for London's colour standards guide, which defines the precise colours from the Pantone palette and also a colour naming scheme that is particular to TfL. Earlier maps were limited by the number of colours available that could be clearly distinguished in print. Improvements in colour printing technology have reduced that problem and the map has coped with the identification of new lines without great difficulty. Pecked lines have at various times indicated construction, limited service, or sections closed for renovation. Station marks From the start, interchange stations were given a special mark to indicate their importance, but its shape has changed over the years. In addition,