Take the bike to the parcel booking department in chennai well 2-3 hrs ahead of your train's departure alongwith all the documents of bike and empty tank. They will weight and pack it. They will make receipt and you have to pay them in cash. After that coolies will be assigned and they will board up the bike in luggage van. At ahmedabad, you don't have to worry. Guard have every detail of luggage that is on-board,at which station what is to be leave. So at ahmedabad, porters will de-board the bike and they will take it to the ahmedabad parcel booking office. Their you show your receipt and get you bike back.