After a final encounter with the Tyrant who is obliterated by a rocket launcher, Leon and Claire must quickly make their way off the premises. A self destruct mechanism balances a sealed fate, and times their departure.
The G5 form was a further degradation of the G4 form. The mouth and digestive system of G4 remained to support the massive nutrient requirements. Whole bodies were consumed by G5, which could eat even Zombies. However, these were digested over time, and partially consumed and recognisably-human biomass covered its surface. The body itself became more mollusk-like in appearance, though propelling by using four tentacles to drag itself along.
An unexpected jolt of the train interrupts everyone's calm. Leon, Claire, and Sherry group into the same train car to question what happened. One of them leaves the party to search other cars of the train. Just when they thought they've seen enough explosions for one day, the train's self destruct mechanism is mysteriously activated. In the process, the train door on its main car is locked off so one of the heroes is trapped from returning. To increase matters for worse, the stray person becomes faced with William who has transformed into his final evolution stage, a large, nasty mass of flesh and stretchy limbs. The cornered person madly fires any available weapons at William until its skin melts away. Turning to face away from the creature, the hero notices William is still very much alive! Meanwhile, the other adult of this bunch attempts to reach the train control room so the brake can be pulled. Unfortunately, there is no access available. So Sherry volunteers without permission to make way through a small control room opening she can fit in. The G-Virus monster known as William smashes through the recently locked door. One person is cornered in the main car, while the other who faced William earlier is now crawling along the train's roof from car to car. Sherry is confused as to what she is supposed to press or pull for stopping this train. The hero atop the train opens a ceiling grate above Sherry, then instructs her on how to stop the train. Sherry complies, the train finally stops, and everyone evacuates out into the open area outside the tunnel. William now more enormous than ever envelops the entire train with his huge mass and at the last second realizes the train is about to explode. The tunnels erupt in flames as everything, including the G-Virus monster is finally vanquished. Leon and Claire congratulate Sherry on her helpful deed. One of them begins to head off quickly, ready to lead their next step towards bringing Umbrella down.