The Nuremberg U-Bahn is the newest and the shortest among the four full-fledged U-Bahn systems in Germany (the other three being Munich, Berlin and Hamburg). Whereas most other German cities that did not already have an U-Bahn by that point in time decided to convert their existing tram network into a partially underground light-rail system (often called Stadtbahn, or - for added confusion - U-Stadtbahn), Nuremberg decided to build an entirely new system from scratch that was supposed to eventually replace the tram system. (It didn't.)
Despite its Non-Indicative Name, it actually also serves the neighboring city of Fürth, but due to the rivalry between the two cities, many people from Nuremberg pretend this to not be the case. The line to Fürth is the Spiritual Successor of the tram line to Fürth, which in turn used the right of way of the former railway line to Fürth - the first ever built in Germany. There is a mosaic in one of the stations along this line that shows this history. The U-Bahn was opened in 1972 after first ideas to move (part of) the tram network underground had already been voiced in the 1920s. As it was partially financed with money from the Bavarian government in Munich, the first generation of trains was built to the same specifications as those for the Munich U-Bahn (and in fact Nuremberg trains saw service for the 1972 Olympics, whereas Munich trains were used in Nuremberg at peak times like Christmas), however the newer generations of trains cannot be swapped around anymore.
The U-Bahn has three main lines (creatively named U1, U2 and U3). With U3 using the same trunk line as U2, you can say that the network amounts to two trunk lines that offer interchange at the two stations Plärrer (Nuremberg's market square, literally "pitchman") and Hauptbahnhof (Main Station). note The maiden stretch of U1 was a kid of its time as it's actually been built outside in for accessing the typical post-war housing projects in southeastern Langwasser. The architecture of the oldest stations is nothing to write home about, but the newer ones are somewhat less cookie-cutter with even some attempts at art and thoughts given to design elements. Some stations have designs associated with their name with Rathenauplatz having a portrait of the Weimar Republic politician murdered by radical rightists.
U1 is the oldest line and was most recently expanded in 2007 with further stops in Fürth, Fürth Hardhöhe being the last. It's other endpoint continues to be Langwasser Süd, the first station ever to be built for the Nuremberg U-Bahn. U2 was started in the 1980s and its one terminus (Röthenbach) dates to that era while the other is the Airport and was reached in 1999. The U3 was only started in the 2000s with the first station opening for service a year after the last extension of U1 and an extension (Nordwestring, the Northern terminus for the time being) coming into service as recently as 2017. As of 2017, a further station for U3 is under construction along its Southern stretch and several more are planned. There is some debate about extensions of U1 in Fürth and U2 towards neighboring towns like Stein. On the other sides of all three lines, the current endpoints (Langwasser Süd, Airport and Nordwestring) are widely regarded as the final termini for the time being. Similar to the situation in Berlin but less markedly, U-Bahn extension is often a political football with the more right wing CSU often coming forward with rather ambitious expansion plans while the more leftist SPD Greens and Left Party tend to advocate for tram extension instead. However, despite the mayor being SPD, he relies on CSU votes for his majorities so usually some compromise is found, making many extensions of both tram and U-Bahn stuck in Development Hell for quite a bit.
Nuremberg U-Bahn holds the distinction of being the first U-Bahn in the world with driverless trains in mixed operation with driver operated trains along the same route. An advert announcing this breakthrough made a rather tasteless pun between "Fahrer" (the German for "driver") and "Führer". Despite its relatively short length (just a bit over 31 km), the U-Bahn has proven remarkably efficient and when Edmund Stoiber (at the time prime minister of Bavaria) gave a rambling and incoherent speech about how the Transrapidnote could get people from the Munich airport to the main station in just ten minutes, a Nuremberg newspaper could not help but make the very humble brag that the Nuremberg U-Bahn already got people from the airport to the main station in twelve minutes. As of the 2011 route extensions, about two thirds of the inhabitants of Nuremberg live close to a U-Bahn station, and the tram lines serve most of the rest. Expansions are currently underway along line 3 (thus far the shortest and youngest) towards the South and West. The metro offers integrated ticketing and several connection points with and to the Straßenbahn (Tram) network, the S-Bahn and regional trains (mostly at Nuremberg and Fürth main station)
While the initial plan of the 1960s was for the Straßenbahn to shut down at some point and many lines - including the historic connection to Fürth and all lines in Fürth were indeed shut down, this was slowly but surely questioned and by the 1990s the city officially reversed the decision and is now expanding the network once more. In order to not confuse riders and because the numbers are free anyway, the Straßenbahn is numbered (without any additional letter) with the numbers 4, 5, 6, 7 and 8. The Straßenbahn has been expanded as recently as 2016 and there are grand plans for expansions all the way to neighboring Erlangen along Line 4.