That's as good a time as any to think about what sort of trains to run under the tree.
The quest in model railroading, though, has been to be able to make the trains smaller, so as to be able to have more action on the top of an end table, or in a briefcase. Thus "Standard" gauge, with its large trains and no end of action accessories, gave way to O Gauge, as in the above, and before the War something called Half O, now HO, became practicable.
When I first became aware of model railroading beyond the Christmas display, the smallest commercially available scale was TT, for Table Top, at 1/120 of actual size. For some reason, that became a popular scale in Soviet Russia, although it was very much a boutique scale in North America.
The quest for miniaturization continued, though, with N scale, at 1/160 of actual size drawing a lot of commercial support from the late 1960s on, and becoming the second-most-commonly modelled scale after HO.
That, however, was still not small enough, and something called Z scale, envisioning the end of the alphabet and the ultimate in scaling things down, at 1/220 actual size, showed up in the early 1970s. One selling point was that a circle of track could be concealed in a desk drawer for an office worker to sneak in some playing with trains when the pointy-haired boss wasn't watching.
The alphabet might have been exhausted, and yet, not the impulse to miniaturize. There's now something called T scale, at 1/450 actual size.
These scales offer all sorts of potential for complex displays under small Christmas trees. I wonder, though, whether operators who like to switch industries will find value for money in them. On the one hand, it's possible to capture a paper mill or steel plant without having to build a barn, but on the other hand coupling and uncoupling cars looks to be a chore.
The miniature motors, though, intrigue as possible ways to motorize O Scale diesel-electric or straight electric cars and locomotives with prototypically accurate drive trains.