We bought the 1-day Thomas pass from Keihan Demachiyanagi Station which costs 850yen per adult. Children 1 to 6 years old are free, just like the regular train fares for kids. But they gave a special big-sized Thomas ticket for kids as a token. You can get the tickets from the train station's information counter. The attendant at Demachiyanagi Station can speak English. The ticket covers a round trip from Demachiyanagi Station to Hirakatashi Station, as well as unlimited ride on the Katano Line.
Every station along the Katano Line are decorated with pictures of a particular Thomas & Friends character. Starting with Percy at Hirakatashi Station and ended with Thomas at Kisaichi Station. Yes, they made you go all the way for the main character :p But for us it was well worth the trip. Even though the attraction is simply big pictures of the characters on the station walls, it was enough to get my son and a few other Japanese kids with their familes excited. Personally, I see nothing interesting surrounding the stations so you don't really have to get out from the platform area. But some stations do put wall panels of Thomas & Friends outside the ticket gate, as well as remembrance stamp, so it's better to get the 1-day Thomas pass to enjoy those.
The train itself is actually a normal passenger train that goes along the line so regular passengers, not only fans of Thomas, would ride it as well. What makes it special is that the train was heavily decorated with Thomas & Friends characters inside and out! You could also hear voices of the characters played as announcement inside the train. There would only be one or two Thomas decorated train in hour. You can get the special train schedule at Hirakatashi Station. What we did was, we just hop on a regular train and went on to visit the next station first to take pictures and just went along the line until we caught up with the Thomas train.
The train design running on Katano Line is called Type 10000. Another one, the Type 700 runs on the Ootsu Line. It seems that these special Thomas trains are yearly attraction done by Keihan Railway, where they would change the train/station design every year. Here's the link for Thomas the Tank Engine No.2011 on Keihan Railway website. The site is in Japanese only.
One of the most common attraction at Japanese tourist sites is remembrance stamp. Keihan Railway is currently running a Thomas & Friends Stamp Rally at Katano Line, Ootsu Line and the Michigan Paddle wheeler Cruise on Lake Biwa. If you don't wish to join the competition, bring along a notepad cause the stamps are worth collecting.
The whole activity takes about 2 hours. This attraction is highly recommendable for a cheap day outing with kindergarten-aged boys.