Sometimes we'd see extra heavy loads on trains. Special depressed flatcars with multiple trucks are used to spread the load. On rail cars, a 'truck' is usually a pair of wheelsets, a center bolster and two side frames to hold the wheel bearings in place. The usual AAR load limit with this arrangement (four axles and eight wheels per freight car) is 286K pounds for the rail car, including the weight of the load and the car itself. Cars that push this limit are usually bulk commodity traffic such as coal cars.
But often, a load needs to be moved that is beyond this weight limit. Then it is necessary to handle the weight, and to spread it out. It isn't just the rail car that takes the weight, you need to also consider the rails themself, the type of road bed, and limiting structures such as bridges. So the answer is often to add extra wheels and trucks to a car to spread the load, and in many cases the load itself is carried by two or three flat cars all tied together.
Here's a heavy load a few years back - 1.2 million pounds! I think it was a cracking tower, it was heading to the Fort McMurray heavy oil fields, originating in Thunder Bay, Ontario. You can see the large structure has a lot of wheelsets carrying the load! Special shipments like this take a lot of calculating, considering maximum dimensions to make sure everything enroute has clearance, optimal speeds are figured for cornering, safe braking distances, available sidings to let mainline traffic pass by and, as mentioned, the ability of the track itself to handle it. 1.2 million pounds isn't as much as it seems, three large locomotives together will push that weight. But the extraordinary size of something like this takes a lot of paperwork.
It's when the load reaches the end of the line, and still has to travel to its destination by special truck that things get real interesting. Check out the trailers that this rig required.
Lifting up off the rail car.....
..and setting down on the trailer.
How'd you like to be this truck driver?