Hint: It's not from an episode of I Love Toy Trains
It's actually Princeton physicists calibrating a nuclear fusion reactor with a TOY TRAIN
Ok, it's not as absurd as it sounds, according to the NY Times. In order to fine tune the neutron sensors inside the reactor, scientists at the Plasma Physics Laboratory ran the train on a circular track for three days inside the reactor, carrying a chunk of californium-252 that released neutrons as it disintegrated.
Previously, neutron calibration had been carried out with a stationary chunk of the same element, but scientists at the lab discovered calibration is 10x more accurate if the element is moving around during the reactor maintenance. The reactor is part of a larger Spherical Torus experiments, which is looking at ways to fuse hydrogen atoms at high temperatures, in a similar manner as the sun.
And for all it's troubles, the train was able to return to it's spot around the laboratory X-mas tree afterwards. But don't worry, californium-252 is hardly radioactive, so everyone was safe. [NY Times]