Most model railway drives use a helically-toothed "worm" that drives a "worm gear" having angled teeth. This design has the advantage of compact size, high reduction ratio, and a 90-degree offset between the motor shaft and the worm gear axis. Instead of a solid machined worm, Tomix in early drive designs used a spring that looks a bit like the spring inside a standard retractable ball-point pen. The open-spaced coils act like the helical worm tooth. The spring also acts as a universal joint, flexing when the the truck/bogie swivels. Because the spring might also flex to climb out of the worm gear teeth, an open-bottom plastic box is used to contain it in place over the worm gear. This arrangement causes the spring to rub against the containing box, creating a potential source of friction and noise. A number of railway modelers have reported terrible luck with the "Spring Worm" drive design, while others report satisfactory operation.
A list of Tomix product numbers through 2011 for powered rolling stock (sold separately or in a set) that use the "Spring Worm" drive is provided here, so modelers can know whether a particular unit has it. Over the years many of these locomotive or powered car series have received "updated" drive designs lacking the "Spring Worm," under different product numbers. At times, concurrent models of the same series have been a mixed offering, with different drive designs used in different product numbers. Do not assume that every Tomix locomotive or powered car in a prototype series listed below has this drive, unless it is marked with a "*." In addition, some prototype series whose models had "Spring Worm" drive have simply been "retired" entirely from the Tomix product line, instead of being updated. The few product numbers which are still in the Tomix product line are shown in bold print. I have tried to make this list as thorough and accurate as possible with given resources, but there may be errors and omissions.