Side trip from Madrid

There is no best. What do you want/like to see? Best for someone else might not be best for you.

I'd pick Salamanca - we took the bus rather than the train. They have a beautiful Plaza Mayor and I'd have lunch there, go to the fun Art Nouveau Museum, and wander around the rest of the town.

I agree, there are no "best"...but three in the order of distance would be: 1/2 hour Toledo. Leave the Atocha station, and 25 minutes later step off in Toledo. Walk out to the right to the street and catch the local bus up to Plaza Zodocover. Explore in any direction. Later in the day catch a taxi back to the train station. 1+ hour Salamanca May be about 1 1/2, I do not recall. There are two stops, the last stop is (I believe) the end of the line. Again, get off and walk almost anywhere. The Palza Mayor is fantastic, supposedly the best preserved in Spain. Lots and Lots of University School buildings, and wonderful places to explore. 2 1/2 hours Seville - Longest journey, and the station is out of town a distance. Would suggest an over night stay at least. The connection with Columbus and the other explorers is amazing! Check out Really Discover Sevilla Tours. (See web site)...Flamenco is here and foot stomping attention. These would be my three recommendations...but others will have equally attractive choices as well.

Thank you both for your replies.
Looking for your opinions, realizing everyone has one. So let me put it this way, if you had only one day to take a side trip out of Madrid, where would YOU choose to go? Thanks in advance.

One day trip only, for sure it would be Toledo. It is one of the more charming towns in Europe and has immense Spanish history and cross-cultural sites. And it is extremely easy to get to for the day from Toledo. Next on my list is Segovia. I think Sevilla is just too far and too big for a day trip.

I know that you said "by train". But if I were limited to one day only outside Madrid, and as long as the Valley of the Fallen is closed, I'd pick a bus (gasp!) trip that goes to at least two towns. Because we could walk to the Trapsatur office from out Madrid hotel, we took (a few years ago) the Avila & Segovia trip, which was memorable. You can save a few Euros if the trip still has a "no lunch" option, eating on your own in the resulting additional free time. (There was nothing wrong with the included lunch, just that it took too long away from touring.) Even the Toledo/Escorial combination may be better than trying to do both on your own by train. I'm not big fan of organized tours, not even Rick's. But I'm simply mentioning a very efficient (if not the cheapest) way to see a lot in one day. My two cents.

For one day -- Toledo..leave on an early train and return late...no doubt

Toledo - about 35 minutes and frequent trains.