Trailers are hauled off the flat cars via a ramp placed at the end of the track. The tractror backs onto the flat car(s), hitches up the trailer, and drives off.
Containers may be moved off sideways with a large fork-lift, or with a specialised side-loader/unloader, or with an overhead crane that runs on rails placed on either side of the contianer car tracks. The crane is the most recent method. Lareg container terminals have been built (eg at Halifax, Nova Scotia, to handle container transfer between ship and train, and between train and truck.
Another way they are handled is the same as containers. They are picked up and set on/off the car by a crane or a huge forklift machine made especially for that purpose. We can see all three methods in use in the Atlanta area. I do not think the drive-on/off method is used around here any more but the last time I was there, the ramps were still in place at one of the local freight yards.
On my HO scale models the trailers are put on/removed by hand when no one is looking. I have found that this saves greatly on the cost of a functional crane.
If anyone sees you you can say that you play god and help your plastic humans :o)
It depends on the era you wish to represent.
Historically, what they refer to as "circus loading" was used. This involves a ramp that abuts the track and the flat car is shoved up against the ramp. A tractor backs the trailer along the string of flat cars to the far end, drops the trailer and moves off again to pick up another trailer. The process is repeated until the cars are loaded.
To unload, the process is reversed.
Very early piggyback service involved about six guys scurrying around the trailer attaching tie down chains, clamps and other such stuff to hold the trailer in place. But someone had the bright idea that if the fifth wheel on the tractor was good enough to hold the trailer in place on the road, why not try it on the flat car. So, now except for special cases, you almost never see a trailer secured to the flat except at the fifth wheel.
Jump forward a few years. Various builders started offering specialized machinery to speed the process up and to eliminate more workers on the ground.
As a result, there are monster machines that are, in essence very big fork lift trucks. A nice model of such a machine is offered by Walthers in HO. It can lift a trailer or container with the blades from the bottom, or use the supplied spreader and clamps to lift one from the top. I seem to remember the kit also includes the arms to life a trailer or container from the bottom.
The final step, at least at this time is based on the "Straddle Loader" used in the lumber industry. These are very big. They straddle the rails and are often several tracks wide. They drop a clamping rig down and lift the container or trailer off, slide it to one side and place it on the ground, or on a waiting tractor or empty container trailer.
At one time, these were usually mounted on a fixed rail system. Brawa offers a ready-to-run machine like this, and it's in the Walthers catalog. It is a working model but I have no idea how reliable it is. Rail Mounted Gantry seems to be the industry term for these things.
More common is the "RTG", or Rubber Tired Gantry which has the flexibility to move around a yard as needed, not being tied down to a fixed guide track. Again, they come in many sizes, and Walthers has a kit of such a machine.
I saw one moving around the El Paso ATSF yard one time, and it was a wonder to behold. About the size of a three story building on wheels, it wavered and wobbled along as it moved from one side of the yard to the other, each leg seeming to move in its own direction, only forming a consensus of where they were going with some dissension...
I came to the conclusion that such an ungainly creature couldn't possibly work, but they are common now in yards that have a lot of intermodal.
I understand the military still uses circus loading even now as they need to be able to get equipment on and off flat cars where they are needed, not where such machines are available, and a temporary ramp is very easy to set up in the boonies.
Fred D.
Try this site for intermodel stuff:
MODELINTERMODAL.COM