The Egg train to the Earth's core, is an invention of E. Aster Bunnymund's, who would secretly help invent the first man-made locomotive several decades later.
The railway train can travel underground to any location on the planet that Bunnymund has excavated and built a track for beforehand, including the center of the earth and later, Santoff Claussen.
The entire train is egg shaped and is maintained by engineer eggs who supply the egg shaped boiler with egg shapped lumps of coal. It barely makes any sound when travelling, just a quiet sort of clucking noise.
The train is shaped like an an egg standing on one end. The interior is also egg shaped, with egg shaped railroad cars and virtually every knob, hinge, door, window, light fixture, and mechanical component.