Fact: The Keio Inokashira line connects Shibuya and Kichijōji, which are some high economy development area. (Idk how to describe). It is noticeable for their colorful livery.
Moreover, due to some history reasons, it is not connected to the Keio Main line as the track gauge is different. You can distinguish Keio Main line trains and Keio Inokashira line trains by their livery: Usually, Keio Main line trains use white livery.
And their Semi Special Express has canceled.
Gorō Inogashira in Kodoku no Gourmet
The gauge difference is likely due to how there used to be many smaller, private rail companies that were all merged into the big three(five+) that exist today.
It would be a colossal task to rip it all out and re-gauge the tracks. And in certain areas of Japan the tight spaces would make increasing the gauge impossible.