Disneyland was a hit. But, after a while, things seem to get repetitive. And of course, Walt wanted to build things he could not build before due to a lack of money.
So the first Major Disneyland expansion was introduced in 1959. The Disneyland-Alweg Monorail, Matterhorn Bobsleds, and the Submarine Voyage were intorduced to Tomorrowland.
The Disneyland Monorial began as a simple elevated train ride around Tomorrowland and was the first daily operating system of it's kind in the western hemisphere. He loved this form of transportation, and soon continued it to the new Disneyland Hotel. A station was built there and people could ride between the hotel and Disneyland. Throughout the years, the monorail would largely reamined untouched while Disneyland grew around it.
Walt was inspired to build The Matterhorn Bobsleds after going to Switzerland and making Third Man on the Mountain. With the new skyway that connected Fantsyland and Tomorrowland, there was a need to hide the tower that held the Skyway up. And, it covered the astetically unpleasing Holiday Hill. Disneyland's Matterhorn is a 1/100th Scale, making Disneyland's 147 feet tall and the park's tallest minature. The Matterhorn became the first tubular steel roller coaster, which would become the most popular form of coasters. In later years, it would officially become part of Fantasyland and become home to the Yetti.
Last but not least, the Submarine Voyage took people underwater to discover "liquid space". Now Disneyland had one of the largest operating submarine fleats in the world. The ride would take guests underwater and underneath the North Pole. Real mermaids were also featured in the lagoon, but were removed when men kept jumping in to get them. The ride closed in 1998, but repopened with a new Finding Nemo theme in 2007.
As you can see, today, all 3 attractions are quite popluar. And just like Disneyland, 50 years later, they continue to delight a new generation of guests, as well as the older ones.
But still, age was cathcing up with Tomorrowland. The "futuristic" year of 1986 was still years away but advances in science were cathcing up fast. Now, Tomorrowland would need a brand new theme.