The way the prototype/carpart/car situation has evolved over time, is complex, and it has been that way for a reasonable time.
The basics are that every car/loco has to have a prototype that it refers to. The Prototype carries information about what sort of car this is (a "loco" versus a "car", for example) has a unique name and is in real-world dimensions (scale neutral). It can then used by a Carpart which is a specialization into, for example, something you might buy (think part number) that has a scale. Then you turn those into the actual Cars you have on the layout (as you might buy several examples of each part), but you can also directly point at the prototype and it will be added as a Car in your scale using whatever settings you would like as a "Custom" Car.
So the prototype is fundamental. There are 7 provided prototype files in the library. You can search on the word "Prototype" to find them in the Library function.
- br.xtp British Rolling-stock Prototype Cars (76 cars inc 33 diesels, 8 steam)
- eu.xtp European Rolling-Stock Prototype Cars (2 cars, no locos)
- protoam.xtp North American Rolling-Stock Prototype Cars (106 cars, inc 61 diesel locos)
- protoam-slotcar.xtp North American Slot-Car Prototype Cars (1 car)
- proto-ng.xtp North American 3' Gauge Narrow-Gauge Prototype Cars (4 cars, no locos)
- protosteam.xtp US Steam Engine and Tender Prototype Cars (4 locos)
You can also create your own prototypes - the ideal is to have a nice drawing of the top of the equipment - but there are defaults. What is needed is knowledge of dimensions and the position of the wheels that constrain movement (pivot points). The model, at the moment, is pretty simplistic - it assumes two pivot points (bogie centers) that can be positioned within the envelope of the car and (as of 5.2) whose central point can be offset from car center (to allow a more realistic "swing" around curves). The idea of pony-trucks, additional wheels and so on is not known to XTrkCAD today. For a steam engine, the bogies are positioned at the start and the end of the coupled wheels. Obviously an articulated loco will pose a challenge as it has two such sections, so in those cases, the centers of the two coupled wheel sets can be used. Luckily, for most cases, when the cars are drawn on the layout the running gear is hidden, so the simple "boxes" mostly work visually.
There are some CARPART files that are auto-created during the build process using these prototypes for North American manufacturers as one example. These include atlascho.xtp. Each of the included parts has a reference to the name of a prototype, but the file does not refer to the name of the prototype file that it was built for!! That's what you ran into.
- As you can see, it is hard to know what to "pre-load" prototypes as it depends on what the modeller is doing -> but it is fair comment that we could include a record in the built files at least that contains the carparts of what prototype file(s) it expects and call out an exception naming it, if that is not loaded when it is added.
- Currently, if no car prototypes are found, there is a message that says "no prototypes" as below -
and it suggests perhaps making some but the only way to get to the "Car Prototype Dialog" mentioned in those circumstances seems to be Manage->Custom Parts at the bottom as below!!
This is definitely non-obvious, I would say that the message should tell you that much at least. I think we used to have a way to do this from inventory, but folks understandably got confused between creating a new CarPart and creating a new Prototype when their real aim was actually to add a Car instead!!
The bigger picture is that it would be wonderful if someone were to take up the cudgels and create more good Prototypes for real-world equipment and then definitely more specialized scale CarPart uses of those that represent things you can buy, so that we could cover more of the RTR scenarios outside NA standard-gauge practice.