Train baggage requirements and cost.

We are four friends wanting to travel from Hong Kong to BeiJing by sleeper Train. We would like to know what the baggage limitations are for carry on. For example can we take a large back-pack and a large suit case per person?
Well there be additional cost and limitations on baggage that will need to be checked? If there will be extra cost, what will the charges be? How many bags are allowed per person? What size and weight are allowed per bag?
Will all our luggage be transported on the same train as we travel on? How secure are any valuables that are transported in our luggage?