Security

You'll likely see a walking vendor or 2 in your travels that will have cheap locks and chains for sale. Often at the train stations. Decide then.

Gave away my chains as I use a cable lock instead..

A chain is always a safety, and easy to get at most major stations. And, if you're traveling alone it will give you a feeling of safety when you leave your compartment or seat to go to the loo.

But, personally I've never used one and haven't had a problem - even traveling in sleeper class. I travel in India with a duffel style bag and use it as a pillow when sleeping, zip side to the wall. I also try to get the top bunk and keep my bag up there when possible. If not, then I just stick my case under the seat with everybody else's bags.

Follow your intuition.

There are clips under the seat for the chain bit. Not heavy really a chain and lock and worth it. Lower berths are more comfortable anyways.

There is no need to carry the chains and locks, you can buy cheap chains and lock during your travel or at the railway station.
If you are traveling in First class then there is no need of that because in that you have your own compartment.
Your tour operator should be suggesting this just for the safety sake. You do not need to worry too much about that.

yeah you can usually find them at stalls in or around most stations - get a number padlock so you dont loose the key

if you want an insight into indian trains sleeper class check out our wee blog video we did on a 47 hour train journey from Kerla to Deli - wouldnt change it for the world I miss indian trains it was a great experience

http://blog.travelpod.com/travel-blog-entries/mandibee/3/1307059718/tpod.html

What does anyone have that is so valuable that cant be carried in a very small backpack to take to the john with you or place next to you like marsha100.
A camera, a laptop, a wallet with money and credit cards, an Ipod,some expensive underwear, everything else in India costs less than half the price you paid for it and if they steal it they probably need it more than you do.

You say 'We' so if there are two of you one can watch the bags when tho other goes to the john etc. so no padlock and chain needed.

I took a light bicycle chain and small padlock. But I never used it on the train. My pack was so heavy, no one would want to steal that monster!

I did keep my pack locked at all times.