Truck drivers are important when you think about it. They are responsible for doing inspections and keeping operations safe.
I believe it. How long does someone usually have to train to become one? 
They provide such important services like transporting goods across land. 
What sort of goods do they usually carry?
Items for retail businesses, raw materials, finished goods, etc.
This may be a little silly, by why are they called "semi" trucks?
Semi-trailer truck
A semi-trailer truck (more commonly semi truck or simply "semi") is the combination of a tractor unit and one or more semi-trailers to carry freight. A semi-trailer attaches to the tractor with a fifth-wheel coupling (hitch), with much of its weight borne by the tractor. The result is that both the tractor and semi-trailer will have a design distinctly different from that of a rigid truck and trailer.
It is variously known as a
useful to be able to stick when it is important to do so.
The Twins's Passing Problem
One part of Edward's branch line has a single track with a passing loop halfway along. The trains are usually short enough to pass each other. This is not the case when Donald and Douglas arrive at opposite directions, each with a long goods train full of china clay. They amaze the signalman when they think of a clever solution to get their trains through.
- This story was redone as A