dates back to the 1930s. The appearance of the network's signs has also been changed and the new signs have been gradually replacing the old ones since November 2014. Rolling stock The Buenos Aires Underground has had among the longest serving and most varied rolling stock in the world, in part due to the network having been originally built and operated by three different companies in its early years, causing incompatibilities between the lines. The network began with uniform rolling stock, with the Anglo-Argentine Tramways Company Anglo-Argentine Line A using La Brugeoise cars (with the exception of a few UEC Preston cars), the three CHADOPyF lines (C, D and E) using Siemens-Schuckert Orenstein & Koppel stock and Lacroze's Line B using Metro-Cammell cars. However, as the lines were expanded and passenger numbers increased, more rolling stock was needed, which made the network less uniform, particularly with regard to the problematic Line B, which used a different electrification system and measurements from the other lines. Line B also became the first line of the network to receive second-hand rolling stock in 1995. Current fleets Since then there have been moves towards greater modernisation and standardisation, with large purchases of new rolling stock from companies such as Alstom and China CNR Corporation, as well as smaller orders of second hand rolling stock, which the government of the City of Buenos Aires claims are temporary measures while those lines are modernised to be able to incorporate more modern rolling stock. Such stock has been acquired from Tokyo, Nagoya, and Madrid. From 2013 to 2019, 468 new cars arrived while 232 received refurbishment, bringing the average age of the fleet to 22 years in 2016, down from 34 in 2014. The network's oldest cars – the La Brugeoise cars – were retired in 2013, while its historically most widely used cars – the Siemens-Schuckert Orenstein & Koppel cars – were to be retired in 2015, though this was pushed back to June 2016 following delays on the opening of Santa Fe station on Line H. While there has been much movement of rolling stock, often in a "hand-me-down" manner from higher frequency lines to less used lines (the primary recipient being Line E), the transition to newer models has gone fairly smoothly, despite shortfalls in some lines following extensions. The Fiat-Materfer cars have been the primary stand-ins while the new trains arrive, though this