We want to book our train tickets in 1-2 months advance before our 42 days tour of China mostly by train to control cost.

We are senior citizens.
WE have planned 42 days tour of China mostly by train to control cost.
We want to book our train tickets in 1-2 months advance before commencing our journey...is it possible?
Which agency can help me & what are there service charges?

Thanks.
Mayur Shah