At first, the locomotive on my train transport truck would be just an interesting load for the truck. As I was building it, though, it became something a little more. I wanted to turn it into a working and running locomotive.

As a child, I loved my LEGO train. For Christmas 1981, my parents and grandparents conspired to buy me a train set, some more rails, an additional wagon and a 4.5V motor and battery box. This was everything that I, aged six at the time, could have wanted. I played with it for years, modifying it to my heart’s content. As an adult Lego builder, however, I drifted away from building trains.

A fair few adult LEGO builders may be on the spectrum. And in the stereotype, LEGO train builders even more so. (They are like Texas compared to the rest of the US: very similar, but the trucks, steaks and hairdos are even bigger). I am a physicist, though, if this were true, it would be nothing I could not handle. Furthermore, some of the friendliest and most talented builders I know are “train heads”, but I am not one of them.

When designing my new locomotive, I realised that I might be able to build it to run on LEGO tracks. And then I could not shake that thought. Because they have to fit inside a child’s bedroom, curved LEGO train track have a fairly small radius. This is why trains in LEGO sets tend to have short wagons and are mostly only six studs wide. To scale with the truck, my locomotive is quite a bit bigger at eight studs wide. It is also quite long compared to the curve radius, so I knew this would involve some compromises. No matter what, it would look a bit silly navigating curves. This is why adult LEGO train builders often build their rolling stock seven studs wide. However, some also use third-party tracks, with a larger curve radius. If mine can run on regular LEGO tracks, despite its size, it should not have any difficulty with larger-radius curves. To enable it to navigate the tight curves, the locomotive’s middle driving wheels have no flanges (like LEGO’s Emerald Night). The little wheels at the front are attached to a so-called pony truck, that can swing from side-to-side. The main concession I made for this was that I made the front ends of the cylinders easily detachable.

In real life, the locomotive has a tender to carry coal and water. So, if I were to build that, it could house the motor. I do not have a fancy 9V train motor from the nineties, that picks up power from the metal rails, nor the metal rails themselves. Luckily, a few years ago, I bought the Lego Powered Up cargo train on a whim (and at a considerable discount), and it sat unopened in my attic ever since. So, I pinched its train motor. This uses batteries. Therefore, the tender also had to fit the associated battery box. At 4×8 studs and four bricks tall, it is pretty big, so it was a tight squeeze. The tender has three axles. As luck would have it, the distance between the outer axles on the train motor is a near perfect match for the distance between adjacent axles on the tender. To allow the tender to navigate tight curves, I mounted the motor such that it could pivot, with the third axle fixed in place. A further concession to functionality is that I attached the rear buffer and magnet coupling to the motor rather than to the tender’s body.

All that was left was to see whether it would work, so I built a little test track. The first trial run ended badly, though. The locomotive derailed and rolled onto its side, shedding bits all over the floor! I tried numerous things to fix this, including swapping the positions of the flangeless wheels. Unfortunately, this caused the ends of the locomotive to swing out widely. Moreover, it did not help solve the issue and more derailments ensued. (By now I had wisely removed most of the bits it kept shedding.) When I finally found the cause, after about an hour of fruitless trial and error, I felt silly and relieved at the same time. It was not at all related to the wheel arrangement. Instead, the spacing between the tender and the locomotive was simply too small. As a result, in a turn, the front corner of the tender simply pushed the locomotive off the tracks. The solution therefore was super easy: increasing that spacing by just half a brick. After that, it ran fine.

The locomotive’s wheelbase is quite long, which causes a bit of friction in the turns. Everything works, though, including the connecting and driving rods and the pony car. This elated my inner six-year-old. I will take the truck and locomotive to a LEGO show at some point, but I will also take the tender along. And perhaps, some of the train-heads will accept me as a (temporary) member of their club and allow me to run it on their layout.