Imagine you have a toy train that you love playing with. Now, imagine there is a bigger version of that toy train that can actually pull real train cars! That's called a locomotive, and the Fairlie locomotive is a special type of locomotive that was made a long time ago.
The Fairlie locomotive has two boiler engines sitting side-by-side, which allowed it to go around curves really easily. It was designed this way because many train tracks back then were curvy, and regular locomotives had a hard time navigating them. The Fairlie locomotive was kind of like a snake, because it could wiggle and move in all kinds of directions easily without falling off the tracks.
The Fairlie locomotive also had two sets of wheels, called bogies, which made it very stable and able to carry heavy loads. This was really important back then because people used trains to transport a lot of things, like coal, wood, and other materials.
Overall, the Fairlie locomotive was a really cool piece of train technology that helped make train transportation easier and more efficient.