Learning about train wheels through the lens of Thomas the Tank engine

We all pretty much “get” wheels, right? From the time we’re babies, we see wheeled objects rolling around us on a regular basis, and it’s not long before the functionality of a wheel and axle can be taken for granted as just another part of almost every vehicle out there. So when my four-year-old recently looked up from his heap of toy trains and wooden track to ask about a particular wheel on a particular train, I had to admit that I really didn’t have an answer. The train in question is known to Thomas the Tank Engine fans as “Emily,” and she stands out among her peers with an oversized wheel each side of her boiler, rather than the more common set of small wheels under the locomotive. Fortunately, behind the expressive faces and somewhat snarky attitudes found on Thomas characters, there is some factual basis for many of the engines, and in this case Emily’s giant wheel does indeed have an explanation.

Which wheels provide power?

To make sense of how this character’s design connects with real life engineering, it’s helpful to embrace your inner four-year-old and familiarize yourself with how train wheels work. Locomotives generally have a battery of wheels, but like a two-wheel drive car, only a few wheels are actually being driven by the engine, appropriately called the driving wheels. The other wheels are there to distribute the weight of the locomotive, and stabilize it as it moves down the track. Wheels in front of the drive wheels are the “leading truck” and those behind the drive wheels are the “trailing truck.” Drive wheels could then be coupled together with the coupling rods you often see on the outside of train wheels, joining them together to share and distribute the power from the engine.

Emily’s enormous wheel

So where does that leave our giant mystery wheel on Emily? Emily appears to be based on a class of steam locomotive called a Great Northern Railway (GNR) No. 1 Stirling Single. The important bit of all that is the “Single,” as it refers to the single, gigantic drive wheel on the side of the locomotive, which received all the power from the engine via a large piston on the outside of the wheel. This design was put into service in 1870, and was a way to balance concerns about speed versus stress on the axle. The record-holding 96-inch wheel could move the train along at a good speed while not needing to rotate as fast as a small wheel would. The catch is that a large drive wheel is slower to accelerate, and only goes fast once it’s had time to get moving. These factors made for a train well-suited for express trips between York and London, generally traveling around 50 miles per hour.

Gordon goes faster

At this point, Thomas the Tank Engine fans might be pointing out that Emily is not in charge of the express trains, as that’s Gordon’s job. Conveniently, Gordon is also based on a real locomotive, the London and North Eastern Railway Gresley A1. These locomotives were first built in 1922, and had six drive wheels to get them moving. They were heavy but powerful, and engineering improvements would eventually get them up to 108 miles per hour over short distances. It’s not clear on my son’s toys, but the drive wheels on this locomotive were all 72 inches in diameter, making them only slightly smaller than what was found on a Stirling Single. The wheels just have a harder time standing out when proportioned on a 70-foot-long locomotive.

Thomas’ torque

While Thomas himself was never referred to as being especially huge or fast, it’s worth noting the utility of this cheeky little engine as well. Thomas was based on the London, Brighton and South Coast Railway’s A1 Class locomotive, often referred to as “Terriers.” These tank engines were often used to shunt cars around freight yards, and had excellent acceleration thanks to their six smaller drive wheels. When not using their torque to push other cars, they were used on branch lines since they could get up to speed quickly between stations, completing routes on tight timetables, just like in the children’s books.

The accuracy of these trains isn’t an accident, as The Railway Series was created by a family of train enthusiasts. Wibert Vere Awdry worked primarily as a clergyman, but he also worked for years on England’s Steam Railway Heritage. Christopher Awdry, the son who Wilbert invented Thomas for, also worked with trains, volunteering on the Talyllyn Railway in Whales between writing more Thomas stories. I don’t recall any stories about Emily winning long-distance races, but clearly there was an appreciation for engineering got these engines moving.

Source: Steam Engine Wheel Arrangements, H2G2