We will have only slightly over an hour on Easter Sunday midday to transfer from our arriving train at Chamartin Station in Madrid to the AVE connection at Atocha Station. Since we haven't been to Madrid before, what should we know in advance to do this?

Slightly over an hour...you are assuming the incoming train will be ON TIME.

I've made that transfer twice (from an incoming Elipsos service from France...LATE arrivals BOTH times).

I would definitely plan on a taxi to do this, especially with a short time interval.

There are trains leaving Chamartin for Atocha every five minutes or less. The trip takes 13 minutes.

I wasn't aware of that train schedule Zerlina has mentioned...I would then opt for THAT given the traffic in Madrid.

beachies7500
Assuming you are coming in a long distance train you will have to go up to the main hall and change to a local train to Atocha(3 minutes)
In the past the long distance ticket (leaving Atocha)could be used to move between Atocha and Chamartin, you can check once in your train, otherwise when you go through the main hall in Chamartin buy a Madrid local train ticket which should not be more than 2euros.

And you'll be looking for a Cercanías train, as they are the local trains that run from Chamartín to Atocha. Even on Easter Sunday, they run every 6 minutes or less, all day long. I think the fare is 1.15 euros.

Do you buy the Cercanias tickets from a ticket machine? Do we need coins?
Just want to be prepared with such short time and unfamiliar territory!
THANKS FOR YOUR HELP~~~

Yes, you buy them from machines that make change.

And if you don´t have change, you can still buy them at the ticket counters in the main hall.

Get on any train heading south towards Atocha. It´s less than 15 minutes.

Rgds, Cova