I think it would be really cool to have the U1 route from Ohlstedt to Norderstedt Mitte as it passed through some of Hamburgs most scenic areas, including the Elbphilharmonie. You could get the DT4 (lots of variations from LHB and Bombardier, mainly motor and interior) and DT4 introduced not long ago. I don't live in Hamburg but when I visited I found the U bahn a really nice rapid system. (I know it's quite a big line)
definitely would love to see that, i also want to see the branch to Großhansdorf but similar to the london underground, i would prefer to wait with that one (for either devices to run faster or the game to be optimized) so that we can have the whole map on the hvv route instead of a few line from one place scattered around multiple dlcs like we do with the rhein sieg nord/ rhein ruhr ost dlcs.
