My attempt to book a seat for two between Madrid and Toledo June 26th returning on June 27th was unsuccessful - is there a problem due to the petrol/strike?Could people be flocking to the trains?

Fred,
In what way we you unable to book the seats? I just attempted for those dates, and it did show availability. I got to the last step before submitting c.c. info.

I just booked Renfe tickets yesterday for early July with no problem.

The renfe site is not working well. I will not be concerned about a ticket to Toledo, there are many trains and thr petrol strike is now over for a week.
Maribel, it is perfectly possible that the Renfe site shows availabity for a particular date and it allows you to go throuhg all the purchasing process , just to state at the end, that there are no tickets or your normaly good credit card is not accepted.
As said before, Iberia looks very good beside the Renfe way of doing things.
Good travels

At present the strikes are over, and not everywhere was "out of fuel"

FRED and car,
I've suffered recently the many glitches of the Renfe site, yes, getting all the way to the "purchase your tickets" stage only to be informed that there are no tickets available. That has happened to me far too many times. However, yesterday I was able to make a purchase without a glitch, for the first time in several attempts. So, yes, I've also been frustrated with the unpredictability of the Renfe ticket purchasing system. My advice is to keep trying, as I have, and if you still can't snag those tickets online, when you arrive at Barajas airport, go to the Renfe counter at T1 (downstairs arrivals hall) or T4 to purchase your tickets. Their hrs. are 8 am-3 pm, Mon.-Fri.

(car, yes, the Iberia site is a snap to use compared to Renfe's new system!)

I've found when arriving at Atocha station with my ticket in hand that often all the morning trains to Toledo have been sold out for that day. Also upon my return to the Toledo station, I have seen the signs of no available tickets for the afternoon departures. It has become a very, very busy route, particularly in the summer. The Toledo trains, in my experience, do sell out, so if you can't book the tixs on line, either call the Atención al cliente numbers given on the web page and purchase on the phone or try to buy your tickets at the airport or go to the station the day before just to make sure.