Official inauguration of the railway was on 16th April 1912. Have a look at this article: euroweeklynews.com/news/mallorca/91763-the-s…
The celebrations started March 9th, when a special exhibition dedicated to the 100 years of the train opens in the Ca'n Prunera museum in Soller. The exhibition ends May 6th.
Have a look at the Ca'n Prunera site:
The exhibition "The Soller Railway, 100 years of travel 1912-2012" will be the first of the celebratory events and will feature an extensive collection of documents, photographs and paintings from the railway’s own archives as well as a number of items, such as old uniforms etc. from private collections.
I think the exhibition is well worth a visit. And the house Ca'n Prunera as well.