@joostboudewijns All I definitely knew was that there would be one or two trains each way. A friend in Santiago who had been up at Ascotan said that I should get to San Pedro, where the railway and highway 21 meet, before 9 a.m. Back in 2012 the road was fully surfaced as far as Chui Chui after which it's dirt. San Pedro was about 80 kms from Calama, which is where I started. I only had to wait 15 minutes or so before the Bolivian vans appeared. I followed them and in getting ahead caught up with this train - the mine empties to San Cristobal. so the two were running about 20 minutes apart.
@joostboudewijns I am going back in a couple of weeks, all being well so will confirm what is running. My contact in Santiago says that it's still two trains a day each way, heading to the Bolivian border in the morning, returning in the afternoon. We shall see.