After London, Budapest has the second oldest metro system in Europe. Now practically bursting at the seams, it is being extended by the addition of Line 4, which will link the two main parts of the city, Buda and Pest. The twin-bore tunnel was planned to be 7.2 kilometers long. Mechanized tunnelling was used to protect the underground sources of Budapest's famous thermal spas. Once complete, the Budapest underground railway will be capable of carrying an additional 600,000 passengers.
Two Earth Pressure Balance Shields – "Bochkor" and "Boros" – started up in quick succession in May 2007 for their drives of 5.5 and 5.6 kilometers near the site of the planned Kelenföldi station. The tunnel alignment passed through fine sandy silt and marl. At shallow depths – in some cases with an overburden of only six meters – the tunnel ran under the city center, the Technical University and the Danube. The EPB Shield S-354 arrived at the target shaft on June 18, 2010, followed by the S-355 on July 4, 2010.