The Pumpkin Car is a train car first seen in "The Origami Car." As its name implies, it is a car filled with pumpkins.
Seven years prior to the events of Book Three, Amelia Hughes and the Steward were in the car searching for orbs to use in Amelia's quest to create a car containing her old life with Alrick Timmens. Unbeknownst to them, a passenger named Grace Monroe was also in the car, and was confronted by the Steward while it was scavenging. Amelia called the Steward off before ordering it to keep looking for things they needed. Grace saw Amelia's number when she opened her pod, but otherwise only saw Amelia's robotic suit she used in her capacity as the Conductor, leading Grace to assume Amelia was the true conductor of the Infinity Train and that the goal of the train was to get the highest number.
Upon learning Amelia was the Conductor she saw when she was little, Grace asks if she remembers saving a little girl in the Pumpkin Car seven years ago. Amelia denies it, suggesting either the event was too minor for her to remember it, or she did not actually see Grace at the time.
While traveling through her Memory Tape, Grace witnesses her encounter with the Steward and the Conductor in the Pumpkin Car, although she quickly realizes her memory of it is tainted by her adolescent impression of the Conductor and Steward.