Ground level seems to be 65 (hit f3 to see your X/Y/Z) so I decided 20 down would be a good amount for the tracks for the subway to run.
Also, I added a script which yells to console when minecraft.net goes down. It runs & alerts every 10 minutes as long as mc.net is down, tells you when it's back up and shuts up...could be increased to 5/6 minute intervals but I thought that may be too spamy.