Planning an Iberian Trip - 3 cities in 6 days

Yes and No. I have flown from Lisbon to Madrid (about a one hour flight - don't forget the time change) and I have taken a train from Porto to Lisbon - but not in 6 days.

I totally discourage this approach, you won't get to even scratch the surface of any of these cities. Have you already made your mind about visiting three cities in six days?

What time of year do you plan to travel?

Three cities--two of them major capitals--in six days is not a good plan. Please don't do it. If you're seeking variety, Madrid will suit. You can stay there the entire time and take day-trips to three entirely different places: Toledo, Segovia and Cuenca. If that's still too much time in Madrid (known mainly for its art museums and the Royal Palace; it's not as immediately fascinating as many other places in Spain), add a side-trip to the historic university town of Alcala de Henares.

Or choose Portugal and visit Lisbon and Porto with a visit to Sintra while you're in Lisbon. That's giving Portugal short shrift, but I think it's a fairly reasonable use of six days. You could also stay in Lisbon the whole time and take an additional side-trip or two.

I agree with what everyone else has said about visiting 3 cities in 6 days. You will hardly see anything, and you will spend most of your time inside airports, on airplanes, inside train stations, and on trains, and getting to and from hotels. This is a waste of precious time.

I like all of Acraven's suggestions. And will add a little bit of tine tuning. You could spend 4 nights in Madrid and 2 nights in Toledo. If you choose Portugal, and decide to spend all of your time in Lisbon, I would spend 4 nights in Lisbon and 2 nights in Sintra.

I heartily endorse kmkwoo's enhancements.