>>990396The original plan was to build a fourth tunnel (D-Tunnel) that looped around the nothern city and connect what today are lines 10 and the southern branch of line 6. Two prepared underground stations for this tunnel exist, but as it is now they will never be used, meanwhile line 6 was integrated into the C-Tunnel (yellow) and line 10 remained on the surface.
In recent years some suggested starting an additional low-floor network including line 10, but the costs of a new incompatible system were deemed to high and instead the surface line will be made accessible with high-floor trains as seen above.
Any extension will always face the problem of building high-floor stations in narrow streets, the most likely one would be connecting it to the Zoo branch of line 11, a line that already existed in the pre-tunnel days. But no actual realistic plans exist for that yet.
Disclaimer: Am actually not from Hannover, only visited a couple of times and following discussions in german rail forums.
