Yep, bleach it is. We use this calculation:
Dilute 1/4 cup bleach for each 15 gallons of tank capacity in 1 gallon of water.
Add this solution to the fresh water tank.
Drive around the block to slosh it all around in the tank.
Let it sit overnight.
Open the fresh water tank drain valve.
Wallaby Dan, (thread hijack!)
I see by your sig, you have N-scale trains. They are SO fun. My DH has several sets & wishes he had a place to set them all up. I may give in & let him take over one of the spare bedrooms....someday.