It's because of railway terminology. In the United States, what the UK refers to as "trucks" are what the US calls "freight cars", so the name was changed. However, sometimes the US gets away with using the word "trucks" when the scriptwriters forget to change the lines. In fact, the merchandise of them is always referred to as "Troublesome Trucks".
This is the same reason why they call the guard a conductor, and points are called switches.