An articulated locomotive is like a really big train with two parts that can bend in the middle. It's kind of like a monster truck when it bends in the middle to go over stuff.
This big train is made up of two parts: the front part is called the "locomotive" and it's where the driver sits and controls the train. The back part is called the "tender" and it's where they put the coal or water to power the train.
These two parts are connected by a special joint called an "articulation" that lets them bend and turn together. This is really important because it allows the train to go around curves and corners much easier than if it was all one big, rigid train.
So, basically, an articulated locomotive is a really big train that can bend in the middle so it can go around corners and curves more easily.