Similar things happened in Vienna. The U6 line is completely different from the rest because it was originally a Stadtbahn and not a subway. The rolling stock to this date cannot be used in automated operation at all, has different electrification (overhead wire and not third rail), has lower platforms and still retains many of the old signage.
It's now a proper subway by all accounts but still retains some of its weird history. For instance there is still a track connecting to the tram network so you can sometimes see a subway train go on the tram network for maintenance: http://www.vormagazin.at/tools/imager/imager.php?file=%2Fmed...
I always find it very interesting when some old things stay around and become integrated into something else and that transition period never really ends. Many transit networks in Europe have history like this and it's fun to rediscover this history.
Originally there was a steam operation on that line as well: https://upload.wikimedia.org/wikipedia/commons/thumb/f/fd/St...
Later they made the line even weirder by running mixed tram and stadtbahn operation: https://upload.wikimedia.org/wikipedia/commons/6/64/Wien-wvb... (tram rolling stock stayed on that line even up to 2008 but were not going into mixed operation any more https://upload.wikimedia.org/wikipedia/commons/5/5c/U6_Hande...)
It's now a proper subway by all accounts but still retains some of its weird history. For instance there is still a track connecting to the tram network so you can sometimes see a subway train go on the tram network for maintenance: http://www.vormagazin.at/tools/imager/imager.php?file=%2Fmed...
The stations still retain much of the old Stadtbahn architecture and show the old signage more prominently than the new one: https://upload.wikimedia.org/wikipedia/commons/thumb/3/39/Me...
I always find it very interesting when some old things stay around and become integrated into something else and that transition period never really ends. Many transit networks in Europe have history like this and it's fun to rediscover this history.