I'm 16 and I remember watching this movie when I was little. In the movie there is a guy, the Villain, who falls into acid at some point in the movie. The Villain turns into a giant reptile/snake thing. The main character fights him at one point on top of a train (?) in the middle of the desert.
Possibly the movie is Dreamscape (1984).
The movie is about a government program to train people to enter another person's dreams. Dennis Quaid plays a good guy who early in the film enters the nightmare of a frightened boy and defends him from a snake monster. But someone in the program is killing people by entering their dreams and terrifying them to death. If you die in your dream, you die in real life. The President of the United States is plagued by a reoccurring nightmare. He dreams he is a passenger on a trolley car moving through a city destroyed by a nuclear holocaust that he has caused. This nuclear wasteland resembles a red desert. The bad guy enters the president's dream to kill him, and Quaid's character enters to protect him, and the two battle in the moving trolley train. The assassin assumes the form of the snake monster, which so terrified Quaid's character in the earlier dream. I don't, however, remember a scene of anyone falling into acid, but it has been many years since I last saw the film.