Do you believe that the old carpet is a good solution?
I would expect that the carpet will act as sponge if it rains. The rain will not go directly into the earth.
I think the carpet will serve its purpose.
This is not a deep-pile carpet.
Very impressive layout !!! cant wait to see more pictures of it , complete with the trains, how many trains will you put in it sir ! ? aw do post more pictures please !!!! 
With trains?
There is no time to put trains on it. I have to use the time of the fine weather to build the railway. It's not finished yet.
Find out yourself, how many work it is to make a working and stable (!) garden railway (...not only loosely tracks...).
Number of trains:
I think 2-4 trains. Some additional trains could wait at the train station(s) and some additional locomotives could be provided at the depot.
