There is no maximum baggage allowance on Portuguese trains but don't overdo it!
In August the trains will be full...full!
There are overhead shelves for light stuff and most carriages have some space at each end for baggage. Also you can usually stuff some something under the seats.
Lisbon - Coimbra - Porto
If you are travelling on the Alfa-Pendular or Intercidades in August you should book at least 24hrs. in advance.
For more info:
www.cp.pt
and my blog
http://barlaventostravelblog.blogspot.com/
where you will find an entry where I provided details of buses to Obidos from Lisbon a few weeks ago.
