I Didn’t Choose The Chug Life, The Chug Life Chose Me.

“How many chuggas to a choo-choo?” – The Internet

I’m not that type of engineer, but I can’t stop thinking about this question, so let’s give it a go. Based on what I’ve seen in Reddit and Twitter threads opinions seem to range between 2 to 12 ‘chugga’s to a single ‘choo-choo’. If you were to ask me for my view, I would go hard with 12. I tend to group my chuggas into pairs and usually hit a good 6 pairs before I let fly with a ‘choo-choo’, because I think you need to EARN it. Anything less doesn’t commit.

But that’s all in the realm of personal preference. What does SCIENCE have to say about this?

Well, first off, we need to define exactly what we mean by a ‘chugga’ and a ‘choo-choo’. ‘Choo-choo’ is pretty self-evident (the blowing of a train whistle), but ‘chugga’ is a little harder to pin down.

Based on the typical motion people make when saying ‘chugga’, as well as me repeating that motion over and over while watching 27 minutes* of steam engine footage on YouTube, a single ‘chugga’ syncs up with the sidebars’ up-and-down motion, which – through steam power – is what propels the train forward by forcing the wheels to turn. Thus, a single ‘chugga’ corresponds roughly to one full rotation of the wheels – the driving wheel in particular – which, in turn depends on the speed of our train and the size of our driving wheel.

Chug Life diag

Once we define our speed and wheel size, we should get a rough ideas of the chugga number for a particular distance, which we’ll also need to define. We’ll get back to that once we figure out how often a train does a ‘choo-choo’.

Due to noise complaints, trains don’t go blowing their whistles around willy-nilly. Engineers need to be strategic about when they deploy a ‘choo-choo’, so – barring railyard operations and livestock/sofas on the track – it happens a lot less than you might think. You could ‘chugga’ for miles and miles before hearing a ‘choo choo’, so let’s narrow our focus. A train would most commonly ‘choo-choo’ as it’s approaching a grade or level-crossing, to warn any car or foot traffic of oncoming doom.  The Northeast Operating Rules Advisory Committee (NORAC), who set the operating rules for railroads in North America, have this to say:

When approaching a public highway-rail crossing at grade and at a whistle sign … with the engine in front, start whistle signal at least 15 seconds but not more than 20 seconds before occupying the crossing. The signal must be prolonged or repeated until the engine occupies the crossing. For multiple crossings, the signal must be prolonged or repeated until the last crossing is occupied. For trains and engines exceeding 60 MPH, the whistle signal must not be started more than ¼ mile in advance of the public grade crossing, even if the advance warning provided by the locomotive horn will be less than 15 seconds in duration. (Source)

Now, these rules are largely for more modern trains rather than steam engines, but they seem pretty reasonable, so let’s run with them – especially since they can fill out a couple of our unknown variables regarding chuggas!


Let’s set our bounds as the ¼ mile specified limit between the train and the crossing, with the ‘choo-choo’ required to start at most 20 seconds before we enter the crossing. We need to figure out how far the train will go from the ¼ mile marker before it choo-choo-chooses to ‘choo-choo’.

So how fast are we going? A turn of the century steam train, like the Flying Scotsman, could hit technically 100mph, but according to the USDOT’s highway-crossing handbook, train speed at crossings can range from 10 to 90 mph, with anything over 79 mph classified as ‘high-speed rail’, so let’s just assume 40 mph – since some steam engines ran on the slower end to prevent track deformation.

40 mph means the choo-choo should be sounded starting in the last 0.22 miles from the crossing, with only 0.03 miles (~158.4 feet) left to accumulate our chuggas before the choo-choo. And that means we’re going to have to determine our wheel size so we can find our how many rotations (or chuggas) it’ll take to hit 158.4 feet before the choo-choo: our “Cn” – Chugga Number.

Problem is trains have so many different wheel sizes. So I’ve compiled here a list of important trains, their wheel sizes, and the resultant Cn. A lot of these trains are fictional, so I’ve used the wheel dimensions of the real steam trains they were based on, aside from The Little Engine That Could and Snowpiercer, which are completely fictional and are based on visual approximation.

Train Wheel Diameter (in) Cn
(Rounded Up)
The Flying Scotsman 80 7
Thomas the Tank Engine 54 11
The Little Engine That Could 52 11
The Polar Express 69 9
The Orient Express 79 8
The Hogwarts Express 72 8
Snowpiercer 134 4

Surprisingly, the chugga to choo-choo ratio does tend to skew to the higher side between 8-11 per choo-choo, which puts my initial number of 12 a little outside striking distance, but in the same ballpark. However, the idea of me pairing up my chuggas has no merit, since the majority of these seem to be odd numbers. So let’s just say we’re all winners here.

Except me. I’m the loser who spent real time putting this all together.

*I didn’t have to watch all of it, but I did.

If you have a ridiculous sci-fi question you’d like me to attempt to answer with real science, please e-mail me at NairForceOne@Gmail.Com, with “[BACK OF THE ENVELOPE]” in your subject line.

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