Instructions
Warrior
Take your horde faction warrior to Stormwind or Ironforge. You must be a high level to get past all the guards and into the tram.
Make your way to the tram area quickly and while mounted. The tram is found in the Dwarven District of Stormwind and in Tinker Town in Ironforge.
Look for the "Deeprun Diver" while riding the train. He will be walking around the bottom of the tank. Type "/tar Deeprun Diver" into the in-game text box and press "Enter" to find him quickly.
Use your "Charge" spell once the diver is targeted to move through the glass and into the tank. Nessy is an NPC -- non-player character -- that should be near the diver. She appears and disappears, and may be hard to find.
Warlock
Ask a horde faction warlock to help you -- you must be alliance for the horde to "attack" you, thus helping you get into the tank.
Take both characters to the tram area, quickly and while mounted. The tram is found in the Dwarven District of Stormwind and in Tinker Town in Ironforge.
Have the horde warlock use the Fear spell on you once in the moving train, which will make you run away from them and, ideally, through the glass and into the tank. Nessy appears and disappears, and may be hard to find.
Rogue
Take your horde faction subtlety rogue to Stormwind or Ironforge. You should be a high level to get past all the guards and into the tram, but as a rogue you can use your Stealth spell to move through the city unnoticed.
Get on the train and target the Deeprun Diver. Type "/tar Deeprun Diver" into the in-game text box and press "Enter" to find him quickly.
Use your Shadowstep spell once the diver is targeted to go through the glass. Nessy should be near the diver. She may be hard to find, because she appears and disappears.