Renfe tickets
 

I have purchased tickets round trip Madrid to Toledo for late May. Without knowing how long we would be in Toledo, I chose a late train back to Madrid. Am I correct to assume I will be able to use that ticket on an earlier train back to Madrid? Assuming Seats are available..

If it's a full-fare ticket probably but a discounted ticket no and you may have to change it before the train leaves. see www.seat61.com for lots on things like that - general info trains BETS-European Rail Experts and www.ricksteves.com. There are buses that run that route too. Trains do get full folks here say. Just ask at Atocha station before leaving.