I had no problems buying an e-ticket online from CP for a trip Lisbon-Porto in September.
http://www.cp.pt/passageiros/en/
Automatic 50% off if you are a senior.
I also did Porto-Vigo in Spain but you have to buy this online from Renfe (although it's a CP DMU that runs the service).
I bought a ticket for the Douro line at the counter in Porto.
If you can fathom the Viva Viagem city e-ticket in Lisbon, it covers suburban lines (eg to Cascais) if you choose "zapping" (pay as you go), as well as Lisbon metro, trams, buses and the cross-river ferries.
http://metro.transporteslisboa.pt/eng/customer-info/viva-viagem-card/
Highlights for me were the old river ferries and skyline in Lisbon, and the murals at Porto Sao Bento station are spectacular (as is Porto generally).
The Alfa Pendular trains can do 220kph but there is not much track where they exceed 160kph.
I bought a nice latte for 1.10 in Lisbon Santa Apolonia station. I think that's the best value railway coffee I've experienced in Europe!
Even the on-board refreshments are cheap (nice pastries!).
Downsides: right at the start, if you use EasyJet, they use a long bus ride from T2 to the main terminal (T1) at Lisbon airport, where you arrive at the back of the queue.
And when you reach the new airport metro station, expect to join another long queue for the ticket machines (no ticket office).
I also found that metro interchanges involved a lot of walking and steps between the platforms, much like the older sections of LU.
But generally, a very interesting and attractive country.
Next time, I'll spend more time in Porto and complete a trip up the Douro line.