Rumi Shimizu (Ai Kobayashi) is the villainess of "The Streetcar's Sudden Stopping Case", episode 6.14 of Case Closed (airdate June 7, 1999). She was a student at Hanayama University and was part of the school's "Railroad Society" with her friends Tetsuya Kawasaki and Masao Horii, with the group specializing in photography of trams.
The three ran into Conan Edogawa and his friends on their latest photography excursion, and it was while Tetsuya was photographing the tram as it approached that the ladder he was on collapsed, sending him falling onto the tracks. The driver hit the brakes and avoided striking Tetsuya, but afterwards, Masao was found dead from head trauma that appeared to be caused by him falling during the train's sudden stop.
But ultimately, Conan discovered that Rumi had actually murdered Masao and set up Tetsuya's accident to stage her murder as an accident. To do so, Rumi set piano wire and hooks to set up a trap to pull the ladder down as Tetsuya was taking photos. When the driver stopped the tram, the evil Rumi took advantage of the moment and struck Masao in the head with Tetsuya's new camera case, killing him. Rumi then re-tied the camera case to the railing and set Masao's body in a position to make it look like he fell and hit his head on the case. Conan was tipped off to Rumi's plot when he discovered one of the hooks she used to orchestrate Tetsuya's fall and noticed how the camera case strap had been tied in only one knot after Masao's death (as opposed to the double knot Tetsuya had put it in).
Conan was able to get investigating detective Kenji Wakai to realize that Masao's death was murder, and he deduced that only Rumi was close enough to Masao at the time to commit the murder. Rumi attempted to deny the accusation, but was ultimately cornered when Conan saw cuts on her hand and the police lab test revealed that unidentified blood and fingerprints were on the camera case. As for motivation, Rumi and Masao had previously been romantically involved, and were even making plans to be married. When Rumi learned that Masao was planning to study abroad in Germany, she asked her lover to let her join him, only for Masao to callously reveal that his father had arranged for him to get married in Germany. Rumi confessed to killing Masao in revenge for breaking her heart, claiming that what he'd done was unforgivable, before being arrested (offscreen).