Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Nr I is a class of articulated six-axle (B''2''B'' wheel arrangement), chopper-driven tram operated by Helsinki City Transport (Finnish: Helsingin kaupungin liikennelaitos, abbreviated HKL; Swedish: Helsingfors stads trafikverk, abbreviated HST) on the Helsinki tram network. All trams of this type were built by the Finnish metal industry corporation Valmet between the years 1973 and 1975. Between 1993 and 2004 all trams in the class were modernised by HKL and redesignated as Nr I+ class. Nr I were the first type of articulated tram operated by the HKL. The design of the Nr I type trams was based on the GT6 type trams built by Düwag for various cities in western Europe since 1956, but the Nr I incorporated several technological innovations that had not been available when the GT6 was designed.