You are most likely to use an Altaria train on routes in Spain that have yet to be replaced by the high-speed AVE trains. They are nonetheless capable of up to 200 km/h, so the service is still pretty good.
How can I book an Altaria train?
If you are already in Spain, then the best option is book at the station with Renfe. However, if you are in the UK and need tickets in advance, the book with Rail Europe.