Model train set

A model train set was a toy consisting of miniature trains, and a railway track which the user could assemble for the trains to move along.

The Doctor enjoyed playing with his model train set. His Eighth incarnation once became depressed about having to keep the set maintained, as previous incarnation had built the set over a period of many years, complete with miniature people, and it was a "miracle of craftsmanship", but the Eighth Doctor wanted a system that wouldn't need his constant attention. He spent days rebuilding the system so that it could function on its own. However, when the Doctor later returned to his train set after an adventure outside the TARDIS, he found that a glitch had caused the entire system to go out of control. While the Doctor began to clear up, he noticed a miniature work crew beginning to put things right. He decided to let them fix things themselves. (PROSE: Model Train Set)

While attempting to assemble a model train set, Sam and Harry found parts for, and accidentally assembled, a Cyberman. (PROSE: Going Off the Rails)