The first picture shows how the standard cattle pot is arranged inside. Trailers come in different lengths, and more modern pots have four axles on the trailer, so the middle is smaller, and the ends are bigger.
I've also heard that no two Wilson (that's a brand name) Cattlepots are made the same. Considering there are thousands of them in use, that's a very interesting tidbit to me.
There are also trailers with spread rear axles (the axles are spread apart, and there are two on a trailer), or closed tandem axles (the axles are all close together, and there are two to four of them). Truckers can haul a certain amount of weight on every axle they have on their truck and trailer. It's 12,000 lbs for the front axle on the truck, 36,000 lbs in Wyoming ( it's 34,000 lbs in most states) on closed tandem axles, which are on the back of the truck, and possibly the trailer. If the trailer isn't a closed tandem axle, it will be a spread axle, and can haul 20,000 lbs. As you add more axles to a trailer, you can haul more weight, but the amount you can haul per axle becomes much more complex to determine.
So, if you have a truck and a spread axle trailer, you can haul 54,000 lbs, which is what my dad can haul.
Truckers load trailers to make sure they are at a proper weight on every individual axle. To help them do this, and to keep the livestock sorted and safe, are two floors and a number of pens/compartment within the cattlepot.
Here's how a cattlepot is generally divided up inside. Some pens you can only put smaller animals in, like calves (the doghouse), and others have removable floors so a trucker can either haul a single deck (decks are like stories in a house) of cows, or two decks of calves (the nose).
Here's the view inside, on the top deck, looking toward the nose. My dad and brother are busy putting the deck in the nose of a cattlepot, so they can haul two decks of calves. Truckers can haul just cows, just calves, or a combination of both. The holes are for ventilation for the livestock, to keep the pot from being blown over in windy conditions, and to keep it light. These trailers are made of aluminum, which also keeps them light.
I turned around, and here's the view toward the back of the trailer. To the left is the door to the doghouse, to the right is the ramp down to the door that leads outside. The floors also have a raised pattern on them to provide traction for the livestock. Sometimes truckers put sawdust on the floor also, to keep livestock cleaner or to improve traction. Cleanliness is a huge issue in trucking. You want the livestock to stay as clean as possible - for sanitation, health and presentation reasons. My dad and brother both wash their pots out regularly. The floors are dirty on this day because it was day two of hauling our own cattle, and we hadn't been near a water hose of any kind.
Here they are putting the deck in the nose of one of their pots. The floor stores within the trailer when you take it out. You take it out when hauling cows so the pen is tall enough for them. When hauling calves you put it back in.
Then they prepare to start filling the pot with cattle, cows and calves, in this case.
Kyle pulled a pin, and a part of the floor drops down, creating a ramp into the lower deck of the nose. You always load the bottom pen of the nose first. If you load the top first, when you open the gate to load the bottom, calves will fall out of the gap you can see in this picture, and that's not good.
At this point the bottom of the nose is filled, and my dad is filling the top of the nose with a specific number of calves, based on their body size and weight. Cattle are like people in that some are tall and thin, others are short and wide, etc... and different groups fill a trailer differently. In addition to working around weight limits per axle, truckers also must fill each compartment based on animal size. You want to have a snug fit, where animal's aren't overly cramped, but are close enough they can brace against each other and won't fall down. If you put too few in a compartment, they are more apt to fall down, or injure themselves moving around.
You also never mix cows and calves in a single compartment, unless they're older calves and there's more room than if all animals are similar in size. Small calves could easily be smashed, and injured or killed, by the much larger cows while in transit.
When they're all in the pen, he quickly lifts the drop down door and secures it in place. Cattle pots have all sorts of handy, space saving design aspects. Some are great, others are hated, but they're all used.
Up it goes. You can see where the other door/ramp to lower compartment is directly under the one he's closing.
Following the nose is the doghouse (the back, on top). This L-shaped pen takes advantage of the space not needed by the ramp that leads to the top deck, and is often used for calves, or sick or injured animals.
You fill each end of the top deck of the cattlepot, then fill up the middle, and largest, compartment last. Then you move to the bottom deck.
The pen extends to the right at the back. It's too short for cows, but calves fit just fine. It's also not adjustable, so unlike the nose, you either haul something short in this pen, or it stays empty.
In they go, then dad closes the gate/ramp just like he did in the nose.
Now that all the shorter compartments are filled with calves, we're ready to fill the taller, larger compartments with their mothers. Here's the view from outside.
Up they go. In the large middle compartment on the top deck is a gate right in the middle. Truckers can either "gate" cattle, which means to separate them into two pens by closing the middle gate, or use it as one larger pen. Weight and cattle size often dictates how this decision is made.
We did not gate the cows on this day, so we filled it up with the carefully counted cows. Truckers also make sure they're always where they need to be to get a cattlepot filled efficiently and correctly. You don't want half the cows to load, then turn around and run off. This wastes time, can cause injury to other cows or humans, teaches the cows a bad habit, and really makes ranchers mad. You can just see my dads feet in the above photo, and his hotshot, which he uses as needed to make sure the cows move into the pen and don't run back out.
As the last cow steps in, he eases the gate shut behind her.
Then he steps down and latches the door shut. If a door were to come open during transit it would be disastrous.
From there, he lifts and pushes the ramp you saw the cows walking up forward into a built-in storage area. Then he unhooks this wing, which was used to create a alleyway up the ramp, and ensure the cows went the proper direction, and swings it out of the way so we can load the bottom.
It has a hinge, and you maneuver it around like so, and say a couple choice words if necessary...
Then secure it to the side of the pot.
Flip up this part of the floor to reveal a ramp to the lower deck,
and swing the gate over, like so. Now they're ready for more cattle.
Now, you'll have to forgive me on this. I had to combine pictures from multiple loads, and these of a load of calves turned out better than those of loading cows on the bottom deck. Typically you would load a truck with either all pairs, in which case there would be cows on this lower deck, or with all calves, in which calves would have been put on the top deck where I showed cows.
Anyway, we gated these calves, so a specific number were loaded into the bottom, then the trucker, and sometimes a helper, go in and close the gate in the middle of the large pen.
Then you fill the back half of the pen with more calves in this case (numbers of cows or calves are always changing based on weight, size, number of axles, etc...)
The gate is closed, and latched.
Then the ramp is dropped back into place. Now the only thing left to fill is the compartment at the very back of the cattlepot.
The trucker moves to this position, so he can close the door after the last cow, or calf, has been loaded. He will drop the door, which slides up and down on track, and secure the rope to the side of his trailer.
Then the trucker heads for the unloading location. The cattlepot is unloaded in the exact opposite order it was loaded, so the back is unloaded first (duh, yes, I know), then the belly, and the bottom of the nose is unloaded last.