Thanks in advance for any suggestions on the quickest way to get from Porto to Lisbon by train.
Happy Trails, Linda
Where are you staying in Lisbon?
You can take a train to Lisbon's Santa Apolonia or Oriente station and get from there to your lodgings by metro, bus, or perhaps taxi or Uber.
This May, going the other direction, I was staying in Lisbon near Rossio Square and walked 5 min to the Restauradores metro station, then took a direct metro train to Santa Apolonia - so you could do that going the other way.
In Porto, presumably you'll be staying near the Sao Bento station near the center. You need to take a short train ride to Campanha station, from which the high speed trains to Lisbon depart. I think all tickets to Lisbon leaving from Campanha would include the ticket from Sao Bento to Campanha too.
Take an IC or AP train.
Here's the official trains site: