I've suggested this before, so please excuse the redundancy.
What about returning to Chama with a couple or three (or more if necessary) cut out of the train at Cumbres. This short train could make it back to Chama in time to make another run up the hill in the afternoon.
The clinker here is the requirement that the numbers of departures from the two terminals can't deviate more than 10%. I assume that is still in the contract language somewhere.
But this scenario would give two short trips a day out of Chama. And even if the afternoon train turned out not to be feasible, it would still provide one short trip for people who wouldn't, or don't want to, spend a full day on the line.