So here's the scenario, a mad scientist has just managed to take over the continent on his campaign for world domination (which continent I will leave to your imagination but for simplicity lets say north America or Europe). While his armies fend off any outsiders trying to intervene he must now consolidate his power before he can continue his conquest which to him means repairing and updating his newly acquired infrastructure. Among the updates is the continents old and outdated railway system, easy for a genius in the fields of physics and engineering. As he repairs and upgrades it he reaches a dilemma when he starts to draw up plans for his super broad gauge intercontinental network. Should he build his network from scratch using two conventual rails or remodel existing double track lines to accommodate his broad gauge network?

A conventional 2 rail network would be simpler to build and it would be separated from smaller standard gauge save for large freight depots and passenger stations which would share the gauge on the same track. It would also allow the lines to be built along straighter and faster routes compared to conventional rail allowing faster access from points A to B and have infrastructure designed to accommodate the size and speed of such trains (for reference the gauge our mad scientist has in mind is 14-16 feet wide). And the scheduling would be more efficient as broader gauges would move at faster average speeds and not have to worry about running into slower freight and passenger train. On the other hand modifying existing double track lines would be theoretically faster as you would just need to refurbish existing track and remodel existing infrastructure rather than build from the ground up. The 4 rails would also allow for better stability at speed and greater weight distribution for the new locomotives and rolling stock. The new equipment could even be made to interchange with smaller 2 rail standard gauge equipment. For example, 2 standard gauge locomotives could haul super broad gauge cars and a single 4 rail locomotive could haul two trains of standard gauge cars. This would allow a broader application of the new rolling stock. And the 4 rail plan has the most room for growth as our intrepid mad scientist plans to do the same for quadruple track lines and maybe even 6 tracks lines.

You may ask yourself "wouldn't highspeed rail networks and super sonic aircraft be better" and you would be right, except that this is a mad scientist were talking about so things like practicality and efficiency are very warped in his mindset. Not to mention he has access to technology that is at least 200 years ahead of anything have and the whole concept in of itself is just plain cool. So what do you think he should go with, super broad 2 track railroad, super broad 4 track railroad, or perhaps some combination of the 2 concepts?

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    $\begingroup$ A true mad scientist would combine ALL the concepts, particularly when mocked, just to prove the fools wrong. $\endgroup$
    – user535733
    May 30, 2023 at 21:36
  • $\begingroup$ Is this mad scientist named Brunel by any chance? Looking for a little familial revenge? $\endgroup$
    – elemtilas
    May 31, 2023 at 3:37
  • $\begingroup$ Also, is 14-16' gauge set in stone? Or is there some wiggle room, like reducing the gauge to 7-8' or so? $\endgroup$
    – elemtilas
    May 31, 2023 at 3:47
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    $\begingroup$ This, very much brings some plans (fantasy?) of the WW2 German Government to mind: the Breitspur $\endgroup$
    – Dohn Joe
    May 31, 2023 at 6:38
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    $\begingroup$ Is he a mad scientist, or a mad engineer? $\endgroup$
    – causative
    May 31, 2023 at 7:14

7 Answers 7


If he uses a pair of existing standard gauge tracks as a four-rail single track, he's going to hit a lot of problems:

  • The most obvious is that the spacing between pairs of tracks has been managed for different reasons to running trains on all four rails. Those include clearance on bends to avoid side-swipes due to the length and width of waggons, clearance required on straights to avoid side-swipes due to the Bernoulli effect sucking trains towards each other and the need to minimise overall width to keep costs down. Your mad scientist is going to have to get all the track adjusted before he can use it for double-width trains and doing the job hastily is likely to cause problems for single-width trains.

  • The less obvious is that there are all kinds of solid objects between the pairs of tracks, such as signal posts, station platforms, and trees. So he's going to have to get the track re-organised for that.

  • A painful problem is the points for switching trains between different tracks. All of that assumes that trains are occupying a single track, and most of it assumes the other track in a paired track is used in the opposite direction. This requires re-designing all the track intersections then re-laying them.

  • Worst, bridges and tunnels have been built assuming that double track has two separate trains running on it, so there will often be supports between the two lines, and the height clearances won't be adequate for super-sized trains. It's very unusual for bridges and tunnels to be built over-size because it increases costs.

  • All the signalling needs to be re-designed to cope with double-width trains taking up two tracks, especially if single-width trains are allowed to run on the same tracks.

  • If he wants double-width trains to be able to run in both directions, he's going to need to lay new tracks anyway. The amount of long-distance standard gauge four-track line in existence is tiny. Being able to run trains in both directions increases both capacity and safety a lot.

Overall, it's going to be a lot easier to lay new routes. These can be super-broad gauge, or pairs of standard gauge. Designing a line that uses pairs of standard gauge tracks to allow running two standard gauge trains as an alternative to the wide trains won't be too hard, and allows all the existing rolling stock to be used on the new routes. Of course, you need more complex signalling to cope with that.

  • $\begingroup$ "the spacing between pairs of tracks has never been important in the running of the existing railroads" is not quite true. There are two main drivers: 1/ increasing the spacing between roads increases the space the line takes up, so costs more to build 2/ more spacing between roads is required on curves as the outer corners of cars project beyond their bogies and the midpoint inside edge projects further towards the centre of the curve. So the spacing is deliberately increased in curves to give clearance and decreased in straights to save construction costs. $\endgroup$ May 31, 2023 at 15:12
  • $\begingroup$ There's also a minimum width just due to the Bernoulli principle (sometimes called "railway sway"). $\endgroup$ May 31, 2023 at 17:38
  • $\begingroup$ Don't forget bridges and tunnels. You're going to have to rebuild all of those unless your super-wide trains are only the same height as normal trains. Plus you're going to have to build a whole extra pair of tracks in most places anyway, if you want your trains to be able to travel in two directions. $\endgroup$
    – N. Virgo
    Jun 1, 2023 at 0:01
  • $\begingroup$ Thanks, points added. $\endgroup$ Jun 1, 2023 at 12:38

Let's think about the physics of rail gauges...

  1. The wider the gauge, the shallower the turn radius. The distance between wheel trucks is involved here, but the simple reality is that the wider the base of the vehicle the greater the drag on the outside wheels when turning. I don't know if trains have ever designed independent suspension and axles, but if you've ever driven in an old-fashioned 4-wheel drive truck (a real 4-wheel drive truck where all the wheels are locked to rotate at the same time) and tried to turn on a dry surface, you'll understand what I'm talking about.

NOTE: @Separatrix points out that unlike automobile tires, rail wheels are conical. This likely increases stability (aka, helps keep the train on the tracks), but it also means the train can take a tighter turn than it would with cylindrical wheels because the outer wheel in a turn has a slightly larger diameter than the inside wheel. That's uber-cool! But it doesn't completely solve the problem. It does mean that my 4-wheel-drive vehicle metaphor isn't as useful as I thought.

  1. As the number of rails your train sits on increases, the weight an individual engine or car could handle increases. This is all about weight distribution. Trains already do this to a degree by adding axles to the wheel trucks, but weight-per-square-meter counts, so there's only so much distribution you can achieve by doing that on just two tracks. See issue #1 for why you can't just add more trucks (it lowers the turn radius). Having two more rails on a wider total gauge would seriously increase the weight-bearing capacity of the train. But, conversely, the wider you make your 2-rail wide-gauge system, the lower the amount of weight you can carry due to the mechanical limitations of widening the trucks.

Note: This assumes we ignore other factors that are equally important when it comes to weight distribution: the towing capacity of the engine(s), the friction of those engines on the tracks, the grades encountered, the weight bearing capacity of the rails, the strength of the tie plates and spikes, the weight bearing capacity of the sleepers and ballast... but you could believably ignore all that as an assumed improvement that came along with the multi-rail wide gauge.

  1. The efficiency of the engine traction-to-rail system. One of the many reasons why motorcycles are so efficient is that the drive train (the whole power transfer from the motor to the surface of the road, not just the chain or drive shaft) is small, small, small. Your wide-gauge rail system increases the distance between the motor and the surface of the rails, lowering the efficiency of the motors. From a practical perspective, the reasons for not building super-duper sized trains are similar to those against building super-duper aircraft carriers. There comes a point where the efficiency of hauling cargo is subverted by the inefficiency of moving it at all.

Yeah, yeah... but wouldn't multiple rails allow me to go faster?

Not as much as you might think. What you gain in stability you'll lose in friction to the rails and wind resistance. It's cheaper from a high-speed train standpoint to have a longer train than a wider train. The only reason bullet trains aren't thinner than they are is the mathematics of center-of-gravity become ugly when the object becomes too much taller than it is wide. In other words, this wide gauge idea is great for hauling breathtaking amounts of freight, but not a useful idea for high speed transport.

But couldn't I get around that by making my super-fast, super-wide train short? As in, a meter off the ground kind of short?

I ask that rhetorical question just in case someone thinks of the idea. I have one word for you, just one word: airfoil.

Besides, in the end, technology is almost always second to economy

I've mentioned many times that too many worldbuilders ignore economy. The economics of operating a business of any kind (no matter who's paying the bill) will always drive the adoption of technology. The coolest anti-gravity fusion-powered train won't be used for anything other than an advertising gimmick until the cost of operating that train (including the consequences of derailment...) become cheaper than that of operating diesel engine trains. A hugely simplistic example is this: if the fancy-dancy engine cost 20X a conventional diesel engine, then it had better haul 20X the load or haul the same load 20X faster (rails permitting, which they aren't) or it's not worth buying.

So, would your business mogul choose a two-rail wide gauge vs. a four-rail multi-gauge? That almost answers itself, but the first question is, would the mogul choose to use a wider gauge in the first place?

The answer is almost certainly "no," but let's ignore that question due to narrative necessity. It's already too late to turn away from the choice! In that case...

  • What are the costs of converting existing track to the multi-gauge vs. replacing existing track to the single wide-gauge?

  • What's the cost of replacing every engine... every car... with the new two-track wide gauge versions?

  • How long can the existing track and assets continue to operate and how much will their value decline due to the inability to haul enough cargo or travel fast enough?

Occam's Razor says economics will dictate a multi-rail, multi-gauge replacement every time

I love Occam's Razor, the Law of Parsimony expressed by a 14th century Monk who was probably trying to impress the abbot with his efforts to get out of chores. In a simplistic (and somewhat disingenuous) paraphrase, it says...

All other things being equal, the simplest answer is usually correct.

Not ripping up existing track will always be cheaper than ripping up existing track. Replacing it directly would require one whale of a good discount from the rail, engine, and car vendors or an equally good increase in transport revenues — which are generally unlikely, assuming the mogul could get the financing to do it in the first place.

  • $\begingroup$ Financing? Who needs financing when you're the undisputed master of the entire continent! Here's my financing terms: you do what I want or I "nationalise" your entire enterprise. $\endgroup$
    – elemtilas
    May 31, 2023 at 3:45
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    $\begingroup$ For point 1. Train wheels are conic sections not cylindrical, the wheel on the outside of the bend is "larger" (at the point of contact) and travels further per-rotation and hence doesn't get the issues that 4 wheel drive cars do. $\endgroup$
    – Separatrix
    May 31, 2023 at 10:21
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    $\begingroup$ @Separatrix I didn't know that! However, it only ameliorates the problem. I suspect the fundamental problem remains. Wider track = shallow curves. $\endgroup$
    – JBH
    May 31, 2023 at 12:24
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    $\begingroup$ @Cadence Actually, that's exactly what I'm saying. Wide track means shallow curves. Narrow tracks mean tight curves. It all boils down to the problem of a rigid axle where one wheel travels distance X along a curve and the other travels distance Y and Y > X. That means you're dragging the outside wheel. The shorter the axle (the narrower the gauge) the less the drag matters. $\endgroup$
    – JBH
    May 31, 2023 at 13:26
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    $\begingroup$ The 4x4 locked diff analogy is solid, train wheels are locked on the axle (effectively all one piece, the wheels only come off to be serviced) and thus always turn at the same rate. It's that plus the conic shape that allow trains to follow the rails, curves and all. $\endgroup$ May 31, 2023 at 14:19

So what do you think he should go with, super broad 2 track railroad, super broad 4 track railroad, or perhaps some combination of the 2 concepts?

Neither of the above, really.

The problem with any form of super-wide railway is that it offers no advantages over standard gauge, but does so at higher cost. At the minimum, the right of way would need to be made wider too. If the track needs to run in a cutting or on an embankment, those would need to be made bigger, as well as any bridges it passes over. Greater stability at speeds is cheaper to achieve by tilting the track; weight distribution is cheaper to achieve by making the train long rather than wide. Coupling standard gauge and super-wide rolling stock on a two-track railway only works as long as both tracks are at a constant separation everywhere, with no inconveniences such as e.g. station platforms or bridge pillars in between them; those would need to be removed and the tracks realigned. All in all, it's just so much hassle for no good reason.

But of course the scientist is mad, so if he feels compelled to do something, then he would be less badly served by a separate super-wide rail network built from scratch.

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    $\begingroup$ While your answer is the most boring, it is also the most realistic. So I up voted it. $\endgroup$ May 31, 2023 at 13:47
  • $\begingroup$ @MennovanLavieren, thank you. Damning by faint praise, just the way I like it ;) $\endgroup$
    – ihaveideas
    May 31, 2023 at 14:31

He will pick "the most complicated option"

Have you ever met a real model train enthusiast? The OP is assuming that a mad scientist in the process of taking over the world is choosing his train track gauge petsonally rather than letting some other person who is a train enthusiast choose.

The only possible explanation is that this is not only a mad scientist but a mad train enthusiast.

Once we have deduced this, it is obvious that the train set is the ends of world domination, not just the means.

He or she (just kidding, blatantly he) may be self aware, or may still believe that paying attention to the paintwork on each platform the same sort of attention as several military divisions really is necessary, but in the ultimate analysis it makes no real difference.

The train system won't be chosen for speed: "My trains aren't slot cars!". Nor for cost: "We're doing it nicely, not cheaply".

It will be chosen for some combination of the following:

  1. How similar that gauge or its proportions are to e.g. 1910s Welsh Railways Ltd locomotives (insert different era here).
  2. Feedback at his preferred online forum, or possibly, club day.
  3. How well the gauge looks with the existing real life 'accessories' like stations, park benches, and countryside.
  4. What consumes an amount of time that is maximised subject to not actually triggering a divorce.

To the non train enthusiast, this looks like making things as complicated (but oddly satisfying to look at) as possible, but that is most unfair.


How about the Breitspurbahn ? That proposal had a 3m track and double-height rolling stock.


Existing double-track doesn't play by your rules

While John Dallman's answer speaks to why a "generic" double-tracked railway (with relatively constant track centers) won't work for your contraption, real double-track/multiple-main railway lines can easily get worse than the "generic" case. Case in point: between Victorville and Hesperia, the main tracks of the BNSF Cajon Subdivision cross over each other, with one track on a bridge over the other. Quoting from the timetable/special instructions for that line:

Rule 6.26—The main tracks cross at the grade separation at MP 39.1 and are designated as prescribed by Rule 6.26 on either side of the crossing.

This isn't the only place this is known to happen either (there are three other places in the Western US alone that I know of: one northeast of the Salt Lake City area, one southeast of Tucson, and one in Colfax Canyon northeast of Sacramento that's particularly notable as one of the tracks is in a tunnel while the other track continues over the top of it at grade). Furthermore, even if you don't have to worry about Main 1 crossing over/under Main 2, there's no hard limit on how far apart those two tracks might be from each other, and there are quite a few places on the US rail network where they are miles apart. (There's also a place in Texas where the two main tracks of a double track section of line cross each other at grade. Think about that for a minute.)


As somebody who likes drawing broad gauge trains: I've scaled down my system's size many times. I started with 5 metres, but that was just ridiculous.

I halved that to 2.5 metres (the width of a truck is either 2.5m or 8 foot, depending on its age, but keep in mind the train is at least a metre wider than its tracks so tram lines still needed the lane they are in to be much wider than the others)

And currently, I do 2 metre gauge, which is actually narrower than Brunel's track gauge was.

So, I would recommend you do about 2.5 metres, up to maybe no wider than 3 metres. That is the gauge that a particular evil person was thinking of building in the past (as has been said by other comments)

A note: people in the answers keep using feet as a measure. Just replace every instance I said metre with "yard" and you'll be close enough! :P

  • $\begingroup$ This answer could be improved by describing why you kept scaling your tracks down. What problems were you encountering at a larger size? As is, it just reads as an opinion without any supporting basis. $\endgroup$
    – Cadence
    Jun 9, 2023 at 3:35

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