Couple of comments about this type of car: although most people who pay attention to trains will see one once in a while, high capacity flat cars are not particularly common in North America. They carry a performance penalty for the railroad: their size means they must travel at lower speeds than most other freight, and they need special routing. The shippers pay for this performance penalty, of course. One note about the use of the real cars: they are certainly used to haul bits of large earth moving equipment, but the fact is that most of the time the equipment is not particularly recognizable for what it will be. A large bulldozer or trackhoe that is recognizable as such does not weigh enough to need one of these depressed center flats to move, and the railroad clearance height is such that anything you can recognize is light enough to ride a conventional flat.
The vehicles that are big enough to require transport on a high capacity flat car, such as mining trucks and such, are manufactured in a way that the vehicle divides into pieces each of which fits within a clearance area of about 10' x 18'. This means that most of the large mining trucks and other very large earth moving equipment seen is not really recognizable as such, as it is shipped in bits that are assembled at the site.
If you're running one of these cars on your route, and attempting to do so as real railroads would, they will always be entrained just behind the locomotive, and the train will move at a speed of no more than about 30 miles per hour. Because of the restrictions on operations, there is a good chance that you will see more than one of these cars at the front of the same train. I've seen as many as 7, in that case, probably 5 different shipments. Because of the operational restrictions, if the railroads have two loads, they'll put them at the front of the same train, so as not to have two trains which have to travel at slow speeds.
Note, too, that the Schnabel cars (like the one from Ocemy) get a special train, and sometimes more than one. I witnessed a move on the UP's former Katy line several years ago, where there was a high-rail truck followed about a half mile by a set of 7 light engines (that is, no load) followed by another high-rail vehicle, and a set of 4 engines pulling two empty piggyback length flat cars, a loaded Schnabel car two more empty piggyback length flat cars, and a caboose (not railroad owned), followed by a final train consisting of a train pulling a two passenger cars, and one of the UP's business cars.
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