UNLIKE some other European capitals which began developing their metro networks in the 19th century, Vienna was a latecomer to the metro scene, not opening its first lines until the 1970s. The network has since been consistently expanded to now reach 74.6km across five lines which carried 444 million passengers in 2012.

This expansion continues apace under the metro's fourth construction phase which commenced in 2010.

The first project to be implemented is a further extension of Line U2 from Aspernstrasse to Seestadt. The work follows a previous extension from Schottenring to Aspernstrasse which provided a third crossing of the River Danube and was completed in 2010, with Stadion station opening in May 2008 ahead of the European Football Championships which were held that summer in Austria and Switzerland.

The latest addition to the line is a completely elevated section which is due to open in October and will have an interchange with Austrian Federal Railways' (ÖBB) Stadlau - Marchegg line at the new Aspern station. This station is located east of the present S80 terminus at Hausfeldstrasse which will also be extended to the new station. Seestadt will serve a new urban development on the site of an abandoned airfield which will eventually house 20,000 people as well as new businesses that will employ 15,000. The whole area is under construction at present and will be completed in stages up to 2020.

A southern extension of Line U1 is another project included in the fourth expansion phase. Initially the extension was planned from Reumannplatz to Rothneusiedl, the planned site of a new urban development. However, due to high costs this project has been scaled back to an extension to Oberlaa which is currently served by tram Line 67 and will be abandoned once the metro extension opens in 2017. Work on the project is underway with the first section from Reumannplatz set to run in a tunnel and reach the surface north of Alaudagasse from which the line will be elevated. In order to retain the option to build a branch to Rothneusiedl at a later date, Alaudagasse station is designed to act as a future junction.

The third project of phase four is another extension of Line U2, this time from the current city-centre terminus at Karlsplatz to Gudrunstrasse, south of Vienna's new main station which is due to be completed in 2015. While construction of the new metro link will not start before 2019, there is considerable uncertainty about this project because the alignment has not yet been decided. There is a possibility that it might be abandoned in favour of tram lines.

Although the Vienna metro is quite young some sections are already in need of major refurbishment. The Reumannplatz - Stephansplatz section of Line U1, which opened in 1978, was the first section to be rebuilt and was closed for six weeks during the summer of 2012 for the work to be carried out. This was necessary to renew the concrete trackbed and to add some new points at Taubstummengasse. Signalling equipment has also been upgraded to the standards of the network's more recent lines.

Line U4 is also showing its age. It was built using the alignment of a former city railway line and suffers from frequent electrical and mechanical failures. However, it has not yet been decided whether a complete closure of certain sections will be needed to carry out repairs.

ViennametroRolling stock renewal is another major element of this refurbishment plan with the first generation of two-car class U metro trains from the 1970s now being withdrawn from service. Replacements are arriving in the shape of 44 six-car class V sets which are being delivered by Siemens with 16 more in the pipeline. One class V set is replacing two class Us and 21 of 62 class U trains currently remain in service. The class Vs are now in operation on lines U1, U2, U3 and U4, which are electrified at 750V dc third rail, with class T and T1 trains, which are in effect light rail vehicles that utilise overhead traction, used on Line U6. To combat congestion on Line U6, a further 20 class T1 cars have been ordered from Bombardier, and when introduced into service later this year and concluding in 2014, will reduce headways on the line from 3 to 2.5 minutes at peak times.

While the City of Vienna presses ahead with the improvements included in phase four, thoughts are starting to turn towards the fifth phase of the metro development plan.

Potential projects include the Rothneusiedl branch of Line U1 as well as a Line U6 extension from Floridsdorf to Stammersdorf. This project has been on the cards for some time but has been consistently held back due to a perceived lack of demand. However, the most important project of the fifth phase could be Line U5.

The idea for this line dates back to the 1960s during initial planning for the metro network. The project involves splitting Line U2 at Rathaus and building two new branches. One new branch would be built from Hernals to Rathaus and then continue on the existing U2 section from Rathaus to Karlsplatz and beyond if the extension to Gudrunstrasse is built. This would then become Line U5. The second new branch could be built from Inzersdorf to Rathaus and then continue on the existing Line U2 to Seestadt establishing the new Line U2.

Plans for phase five are still under discussion and may be changed or even abandoned depending on financial support. To date the City of Vienna has received 50% of the funds required to build its metro lines from the Austrian state. Financing agreements exist only for phase four, so phase five would require new negotiations with the Federal Ministry of Finance. But with Vienna backing its metro network to deliver the mobility required by its citizens of today and tomorrow, it will be a strong position to secure further funds.