Hi Patrick
The bus times given by the Chillin Hostel should be accurate as they are there and up to date with the times and options.
I have visited the Chillin Hostel and it is a great location in the heart of Las Trancas and looks very comfortable.
The International Hostel is also on the main...
All the times are on their website http://www.efe.cl and you can book online (although not sure form overseas)
There are just a few trains a day and they do fill up often.
From Estacion Central in Santiago you can buy tickets if there is any space. If not there is a 'Tur Bus' terminal next door...
Anyone heading down to Termas de Chillan should try and book your train tickets before you go as the trains fill up close to the departure dates...
http://www.efe.cl
Las Lenas is indeed an excellent resort for backcountry providing the Marte lift is open,although it shuts frequently.
My personal favourite is Termas de Chillan with great backcountry options and several off piste areas to explore.
Don't overlook some of the smaller resorts that only have a few...