ACE train from Madird to Seville
 

Does anyone know if you need to reserve ahead of time the high speed ACE train from Madrid to Sevilla?

I did that route a few years back and did not reserve a seat.

To my knowledge reservations are obligatory before boarding! Easy to do i think but can't just board train or could be turned away if full or perhaps have to pay penalty fare for doing mandated reservation onboard.
It's the AVE train (alta velicidad e? express??) a French TGV train that goes up to 186 mph.

Bookings are mandatory, and, if you are travelling on a rail pass, there will be a substantial 'supplement'to pay. Been there recently, and done exactly that!

It's AVE (Alta Velocidad Española). These are amongst the very few European trains with check-in desks at stations, so you cannot board the train without a reservation.