Wheels – they’re an integral part of every model train. But you might be surprised to know that there is far more to them than just watching them roll. With multiple size and material choices, things can get confusing very quickly, especially when it comes to replacing them or building a kit.

In this article, we’ll explore what it all means!

The Basics

Wheels are one aspect of model trains which function exactly like they do on the prototype. All types are built with a wide tread which rides the top of the rail, and a lip, known as a flange, which rides the inside of the rail. This ensures that the train is effectively steered by the rails, and won’t come off the tracks. All standard gauge wheel sets must have the scale equivalent of four feet, eight and a half inches between the flanges in order for them to ride the tracks correctly.

Size Matters

It will probably come as no surprise that not all wheels are created equal. Any eagle-eyed railfan will notice that locomotives generally have larger wheel diameters than the cars which accompany them. This is most drastic on steam locomotives, but also applies to modern diesels. If you’re looking to replace wheels on a diesel locomotive model, then 40′ wheel sets will be in order. What might come as more surprising however, is that there is also variation between the wheels on rolling stock.

Passenger cars generally have the largest diameter after locomotives. While these can vary depending on the car, most utilize 36′ wheels. When it comes to freight rolling stock, things start to get a little more complicated. 33′ wheels are the sweet spot. These will generally look correct on most models based on equipment from the 1970s or earlier. Fast forward back to today though, and there is a little more variety. Modern heavy-load cars such as covered hoppers and 60-86′ boxcars utilize the same 36′ diameter as passenger cars, while some special load cars utilize wheel sets as large as 38′. These include coil cars and Flexi-Flo hoppers. On the smaller end, Intermodal Well Cars, Autoracks, and TTX Flatcars generally use a 28′ diameter, to allow for a lower profile for extra height loads.

How Deep?

Until recently, models could come with flanges of varying depth. Now you might think that the deeper the flange, the better the ride, but it turns out that this can actually create more issues. In the past, Code 100 was the standard rail height in HO scale. This was a little higher than most prototype rails, and is now most commonly used only in train sets. As the hobby has become more precise, Code 80 has emerged as the more realistic mainline rail height. As a result of this, cars with deeper flanges which had no issues navigating Code 100 rails, suddenly began hitting rail joints, switches, and would even bottom out and derail while traversing the newer, more realistic track.

In response to this issue, the National Model Railroad Association developed the RP25 wheel specification. Standing for “Recommended Practice”, the RP25 was designed for a more realistic appearance, and optimum performance on all rail types. Today, the RP25 wheel is the standard size used on all North American models, and some clubs even require them. To learn more about different rail heights, check out our video on the topic!

Material Isn’t Immaterial

If your rolling stock fleet dates back beyond the last few years, you’ll likely have a selection of cars with plastic wheels installed. Some cars are still produced with these today, but far more are being released with metal wheels.

While other aspects are important to consider, the choice of going metal or plastic is much more discretionary. The main advantage to metal over plastic is of course durability, especially over time. Plastic wheels are easier to damage, and will warp more easily, while metal wheels are stronger, and have the undeniable advantage of looking much more like prototypical steel wheels. Metal wheels don’t come without their problems however, as a derailment is likely to short out the electrical current on your layout. Generally though, metal wheels are preferred today, for their appearance, durability, and smoother ride quality.

That just about does it for the basics of wheel facts. While there are more intricacies we could get into, such as tread width and era appropriate designs, this should be enough to allow you to get started on your wheel replacement or kit building process. Happy railroading, and smooth tracks ahead!

Explore our selection of trucks and wheelsets in all scales here

Diagrams from the National Model Railroad Association: www.nmra.org/beginner/wheels-metal-or-plastic