I am not aware of any "santa" or "christmas" train on the mainline (standard and narrow gauge) networks.

However, there is a "fairy tale tram" in Zürich and Bern:

The classic is the Märlitram (fairy tale tram) in Zürich: [www.stadt-zuerich.ch] . Beginning around November 23, the Märlitram runs every 20 minutes on the "typical" circuit Bellevue - Bahnhofstrasse - Bahnhofbrücke - Limmatquai - Bellevue. The vehicle is the oldest one of the fleet, being built in 1913, and converted to its purpose in 1958 (hey, that's 60 years ago). It is sponsored by the Jelmoli department store, and except of Santa, the driver, and an angel reading the fairy tale, no one older than 12 is allowed on board.

Bern is also using one of the oldest vehicles in the fleet, with a much more modest livery: [www.facebook.com] . Here it is sponsored by a leading local newspaper.