From Bulbapedia, the community-driven Pokémon encyclopedia.
Neon Town (Japanese: ネオンタウン Neon Town) is an anime-exclusive location somewhere between the Kanto Safari Zone and Cinnabar Island. Its only anime appearance came in The Song of Jigglypuff.
The town is very similar to Las Vegas, Nevada. It is referred to as the "City that Never Sleeps," (which is actually one of New York City's many nicknames) and it appears to live up to that name. While Ash and his friends were traveling through it, a man bumped into them. He blamed the kids and nearly got into a fight with them. Luckily, Officer Jenny was able to break it up before a fight developed. However, she reprimanded Ash and the others, despite the fact they did nothing wrong.
Team Rocket also had an encounter with the man who bumped into Ash and the others. This time, he actually got into a fight with James. When Jessie backed away, she bumped into a woman who began beating her up.
Eventually, Ash and the others found a Jigglypuff who wasn't able to sing. The kids were able to help it, but they wanted to test its ability on a large group of people. They brought Jigglypuff to Neon Town and had it sing. Sure enough, everybody in town fell asleep. The next day, the man who bumped into Ash and the others apologized to them, saying he was just tired.