I would say Kintetsu Utsube Line or like Marinemania said Yokkaichi Asunarou Utsube (5.7 km) and Hachiōji line (1.3 km) which Yokkaichi Asunarou Railway has taken over this year.
Second would be Tsurumi line.
Railshuttle: which part/section of the San'in main line? The line is 676km long.
Pagroove: expensions towards Shinagawa/Yokahama from Tsurumi line would indeed be possible, you might even be able to get Shinagawa station from Andi for when he releases his Yamanote line, but that would also mean his route would be required which will be payware.
About my route I have finally been able to fill an large empty area, remember the map picture at post 1961, page 131?
The large red area above Iwayama depot was supposed to be the depot at the start when I build the route, but I desided to put the depot at the end and extend the table and because of that the area kept being empty till an few days ago, the result you can see in the next pics.
The lake between the tracks going towards Higashi with the 117 driving towards Iwayama station.
The top of the hill.
The second larger lake inside the trackcurve with the depot to the right.
The other side of the second lake where an double brown DD13 starts driving into the depot.