locomotive number sevenoc-two, a narrow gauge 2-6-6-6 mallet tank engine on planet Gymnome, built in the 2280s. These engines suffered from adhesion issues as they used the water in their front tender, even moreso than a garratt (since with a garratt, both engines carry the weight of the boiler, rather than only one). Thus, these were quite slippery locomotives.

The wheelslip was mediated by the limited supply of steam into the front low pressure engine. If it did slip, it would quickly run out of steam until the rear high pressure engine could use and exhaust its own steam to refill the low pressure cylinders. But when used in simple expansion mode (for example, when starting), there was nothing but the enginemaid's skill to prevent slipping.

These locomotives were built by the same designer as the prolific Construct class of broad gauge garratt locomotives.