There have only been 3 + 2 official monorail sets by Lego. Three monorails:

And two boxes of extra tracks:

(For the track boxes, the theme is mainly influenced by the monorail set they were released together with. The tracks do not have anything that makes them special for one theme or another.)

Other than these, a couple of single replacement parts could be ordered, as well.

The above three monorail sets have several features in common:

  • They contain a rather long (especially the Airport Shuttle, but also the Unitron monorail) track.
  • They contain a motorized train.
  • They contain two full-fledged stations.
  • In the case of the Space monorails, they even contain an extra vehicle.

As a result, all three sets were among the biggest and thus probably most expensive ones in their respective theme.

This always struck me as odd: When I was a kid, I loved the Airport Shuttle, but I was unable to buy an extra station or train, because there simply wasn't any. Likewise, the Airport Shuttle was the only monorail set I happened to get1 - and I presume there are plenty of children who would have liked some monorail, but never had a chance to get any of these really huge sets due to their sheer price.

In particular, I contrast this with Lego trains, which have always been offered in quite a different way:

  • There have been train sets of different sizes, in particular "starter sets" that only comprised a small oval of tracks and a short train.
  • Train sets that brought trains would usually only include a rather minimalist station (for passenger trains, often just a simple platform that did not even span the entire length of the train), thereby keeping set size a bit down.
  • Single train cars and engines were offered as separate sets.
  • Stations of different sizes were offered as separate sets.
  • At least in earlier years, there were also completely unmotorized trains as an alternative to the motorized ones, which further helped reduce the set price.

Hence, I am interested in the following: Is there any knowledge on why TLC decided to use a different pattern for monorail sets, by only offering very few, very big sets without any real extension sets (save for tracks)?

1: Although, I admit, this wasn't entirely the fault of the effect I focus in this question. The Futuron monorail was simply a tad too early and I "missed out" while it was still available, whereas I considered attempting to get the Unitron monorail, but ultimately did not like the set as it was designed - what's the purpose of that train (maybe that makes for a good separate question), transport its own cockpits? That seemed pointless, so I discarded the idea, hoping for the next monorail set ... that never came ...


Why design the core monorail hardware this way?

Sariel monorail diagram

This is hard to answer without a degree of speculation, but I assume that the core reason behind the decision to design the monorail this way likely came down to cost and engineering challenges. TLG wanted to allow for a monorail train to be able to navigate inclines effectively. This required:

  • A cog system
  • An additional degree of freedom between cars compared to trains (vertical movement as well as lateral)
  • An additional degree of freedom between wheel sets and cars
  • More specific power to weight requirements to maintain speed on inclines

Ultimately, TLG landed on the fairly rigid system of a central motor and couplings that create the necessary degrees of freedom with a car on either side. This likely required the smallest number of new parts and showed implicitly that the monorail motor isn't designed and tested to work well pulling more than two cars.

Why offer only large sets and not just a basic oval?

The core features of the monorail system that differentiate it from the regular trains of the era became:

  • Inclined and elevated track
  • Stopping at particular track segments rather than being controlled remotely

Given these features, it makes sense that any complete monorail set would have at minimum:

  • Elevated track including ramp sections
  • At least 2 monoswitches. Technically, you could just have one, but that would really limit play value since the train wouldn't have any destination other than its starting point
  • A point of interest at the monoswitches for play value

Unless these features are included, the set might as well just be a regular train set. While you are correct that the later sets were more complicated, 6990 was basically just an oval of track with a pair of ramps and two monoswitches. I believe it included just two straight track sections in addition to these.

Given the inflexibility of the overall system (e.g. you can't just add one more car), TLG decided to include both cars in each complete monorail set and not offer any as add-on sets.

Comparison to 9V train sets

While there were many small train-themed sets in this time period, I'm not aware of low-cost starter sets. You could certainly buy a pack of 9V curved rails and a motor and work your way up from there, but the three complete sets released between 1989 and 1994 were actually fairly expensive:

  • 1991 Metroliner (4558) - $149
  • 1991 Load and Haul Railroad (4563) - $120
  • 1994 Freight Rail Runner (4564) - $134

Compare these to the complete monorail sets of the same period:

  • 1989 Monorail Transport System (6990) - $155
  • 1994 Monorail Transport Base (6991) - $178
  • 1990 Airport Shuttle (6399) - $140

The difference in price between the cheapest monorail and train set from the era is only $20.

Why no add-on sets?

When I was a kid, I loved the Airport Shuttle, but I was unable to buy an extra station or train, because there simply wasn't any.

While there weren't sets that included additional cars in the same way as regular trains, the monorail sets were generally part of larger themes that had additional sets available and were marketed together. For example, there were aircraft of different sizes and an airport marketed alongside Airport Shuttle:

6399 in catalog

The Futuron theme had many sets available as well, including another base:

6990 in catalog

Just in case my answer didn't make this clear, I really like the monorail theme. However, while being great collectors items, I believe that the monorail theme is generally considered to be a business failure. Offering two completely separate train systems at the same time with slightly different features ultimately didn't pan out well for a lot of the reasons that you cited.

  • Thank you for the extensive answer. Just a few comments: I am aware monorail trains are not as flexible as regular trains when it comes to adding cars. Even so, I would have hoped for at least an additional three-piece standard train (motor + two cars) as an extra set. Also, note that new monorail parts were designed later on, at the time of the Airport Shuttle (the switches, 2890 and 2889), and seemingly just "for variation", so an alternative, shorter car base may not have been totally impossible. ... – O. R. Mapper Nov 22 '16 at 20:19
  • ... You are right in that 6990 indeed featured just a rather short track. I also agree there must be at least two stops, but I find it questionable that in all cases, each of these stops was so large it could have been a good space base/town monorail stop set in its own right (compare again the minimalist platforms often included in train sets). Indeed, all train sets after the discontinuation of motorless trains were rather expensive, but at least you could buy an engine like 4551-1 (or even just 4546-1), tracks, ... – O. R. Mapper Nov 22 '16 at 20:31
  • ... some cars and a station separately. To adult collectors, it may not matter, but for a child, it can make a crucial difference whether there are several distinct sets (that are used together, but are bought by different people, e.g. parents, grandparents, or (for smaller sets) friends), or whether there is one huge set that all these people have to team up to buy together. Lastly, I don't quite buy the explanation that the monorails were embedded in other themes - that reasoning would work equally well for offering only a single, complete train set and saying it's embedded in town, anyway. – O. R. Mapper Nov 22 '16 at 20:34
  • I agree that it would have been nice to have separate cars available. The motor was actually available as a service pack (5040), but it's really only useful as a replacement for a lost/broken motor without the cars. Regular trains were designed and sold as a much more flexible system. – jncraton Nov 22 '16 at 21:11

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