Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

The first railways in Thuringia had been built in the 1840s and the network of main lines was finished around 1880. By 1920, many branch lines had been built, giving Thuringia one of the densest rail networks in the world before World War II with about 2,500 km of track. Between 1950 and 2000 most of the branch lines were abandoned, reducing Thuringia's network by half compared to 1940. On the other hand, most of the main lines were refurbished after 1990, resulting in improved speed of travel. The most important railway lines at present are the Thuringian Railway, connecting Halle and Leipzig via Weimar, Erfurt, Gotha and Eisenach with Frankfurt and Kassel and the Saal Railway from Halle/Leipzig via Jena and Saalfeld to Nuremberg. The former has an hourly ICE/IC service from Dresden to Frankfurt while the latter is served hourly by ICE trains from Berlin to Munich. In 2017, a new high speed line will be opened, diverting long-distance services from these mid-19th century lines. Both ICE routes will then use the Erfurt–Leipzig/Halle high-speed railway, and the Berlin-Munich route will continue via the Nuremberg–Erfurt high-speed railway. Only the segment west of Erfurt of the Frankfurt-Dresden line will continue to be used by ICE trains after 2017, with an increased line speed of 200 km/h (currently 160 km/h). Erfurt's central station, which was completely rebuilt for this purpose in the 2000s (decade), will be the new connection between both ICE lines. The most important regional railway lines in Thuringia are the Neudietendorf–Ritschenhausen railway from Erfurt to Würzburg and Meiningen, the Weimar–Gera railway from Erfurt to Chemnitz, the Sangerhausen–Erfurt railway from Erfurt to Magdeburg, the Gotha–Leinefelde railway from Erfurt to Göttingen, the Halle–Kassel railway from Halle via Nordhausen to Kassel and the Leipzig–Hof railway from Leipzig via Altenburg to Zwickau and Hof. Most regional and local lines have hourly service, but some run only every other hour.
When was Thuringia's first railway built?