During the holiday season, many people place toy trains on circular tracks beneath their Christmas trees.

This month, at the Princeton Plasma Physics Laboratory, physicists and engineers built tracks inside one of its fusion reactors and ran a toy train on them for three days.

It was not an exercise in silliness, but in calibration.

The modified model of a diesel train engine was carrying a small chunk of californium-252, a radioactive element that spews neutrons as it falls apart.

Image TRAINED Sly Vinson in a fusion reactor at a Princeton laboratory with the toy engine that was modified and run in circles carrying a radioactive element. Credit... Elle Starkman

“We needed to refine the calibration technique to make sure we are measuring our neutrons as accurately as possible,” said Masa Ono, the project head of the National Spherical Torus Experiment.