Mattia's place is my favorite part of Madrid, the Barrio de las Letras. It's right by lots of good tapa bars and cute little shops on mostly pedestrian streets.
Depending on which streets you take, it's about a 10-15 minute walk from the Atocha train station. Be prepared for the slight slope on the way up if you're coming from Atocha -- it's especially noticeable when you're dragging luggage behind you.
Mattia, the host, was extremely communicative regarding check-in and check-out. Every time I messaged him, he responded within seconds.