First of all: thank you all who replied to my post. All- correct.
Here is an official response I received from the PeruRail rep:

" Dear Passenger,

Thank you for your communication and choosing Perurail. We will provide you the facilities to carry your extra luggage,actually the problem is not the weight but yes it is the big size of the luggage because the trains are panoramic and it has a small space for them , however do not worry, we will giveyou the facilities, I would appreciate if you could send me the number of your train tickets or the sales code and the approximate size of all the luggage.

The cost of this service is included in your train ticket, do not worry, you can send me the information until 48 hrs before the travel date."