If you went in September they are running regular trains from each end. At least that is what is announced, but plans could very well change, as this year is minute by minute for everyone!
If you are going in August you could still do the things you want to do....Chase from Chama, ride from both ends. Thats the perfect scenario really. Riding from Antonito is nice listening to the engine all the way to Osier, and you get the best Gorge scenery both ways. Or you can eat lunch and sit back and nap with the clickety-clack back to Antonito. Chama to Osier is good cause you get the climb to Cumbres, and on the return from Osier you get the climb to Cumbres from the east. Either way you should be fine. Doing two trips or more you might consider a Seasons Pass!!!
Greg