I know how to access underground mode but am not sure what type of tracks & trains I need to use or if there is a special depot for underground mode ? ;D
You build underground tracks the same way you would overground except:
1) use the tunnel icon rather than the track icon
2) build in underground mode
Depot, trains and stations are all the same as above ground. You can connect the two as well - if you don't want to build a full (2-entrance) tunnel you can ctrl-click when you build the tunnel entrance and it will just build the first couple of tiles of the tunnel.
Same here, too: http://forum.simutrans.com/index.php?topic=3933.msg38413#msg38413 ;D