What's wrong with monorails

© 2005, Marc Dufour, all rights reserved

Conventionnal railroads have existed in their present form for more than 500 years (2000 more if one looks at the roman cities ruts).

Once in a while, some people pretend to improve rail transport by proposing radically different systems, one of the most frequent of those being the monorail (which has only one rail opposed to the two rails of conventionnal railroads).

But monorails are far from being widespread, simply because of their inherent general inefficiency.

Granted, for several precise applications, monorails are very efficient, but those are under very limited circumstances, like an industrial overhead monorail used within a building.

Network efficiency

A railroad network is practical when it allows to be networked, that is, when severail rail lines are combined.

Combining severail rail lines means that there will be several tracks which will be interconnected at junctions. Junctions need a way of having vehicles go from one track to the other; this is usually done through switches.

It is plainly obvious that the network flexibility will be directly proportional to the number of switches. In return, the complexity of individual switches will directly influence the number of switches that can be installed, and thus, the very efficiency of the rail network.

The main problem with monorails is precisely the complexity of their switches.

Birail switches

Figure 1–Birail cross-section

A bi-rail system ( henceforth “birail”) has, as it's name indicates, two rails (C) on which vehicles, made of axles & wheels (B) and a carbody run.


Figure 2 – The development areas of birail components

The development areas of each compent is the area swept during normal system usage (excluding surface swept during accidents, such as collisions and derailments).

Development areas are as follows:

One will notice that those development areas intersect.


Figure 3 – Birail development areas intersection

The intersection of development areas of the rolling stock and of the rails are indicated in red on the figure at right:

The smoothness and the safety of the ride depends on the continuity of the rail rolling surface.

The rolling surface cannot be interrupted for level crossings or switches, or if it is, it will have to be within the limits in which such an interruption is not dangerous or uncomfortable.

One will notice here the smallness of this area, compared to the vehicle's area or even the track's area. The corollary is that a birail system will be able to work even when the rolling surfaces will be momentarly interrupted, or that it will be very easy to move rolling surface parts that will hinder the movement of vehicles in the other direction possible.

Let's perform the same analysis on a monorail.

Monorail switches

Figure 11 – Monorail cross-section

A monorail has, as it's name shows, only one rail (C) on which vehicles, made of wheels (B) and a carbody run.


Figure 12 – The development areas of monorail components

The development areas of each compent is the area swept during normal system usage (excluding surface swept during accidents, such as collisions and derailments).

Development areas are as follows:

One will notice that those development areas intersect.


Figure 13 – Monorail development areas intersection

The intersection of development areas of the rolling stock and of the rails are indicated in red on the figure at right:

Comparing with figure 3, one will notice that the intersction area is enormous; almost a third of the vehicle cross-section is intersected by the track development section!

The direct consequence is that switches will have to move a very large surface when switches are turned, this move will result of completely replacing the whole track on a distance necessary to clear the development area of the vehicle as it goes through the switch.

This means that the faster the switch is designed for, the longer the mechanism will have to be.

In the case of a birail, the mechanism is the switch points. Given the tiny surface of rail/vehicle intersection, this mechanism can have a small size compared to the whole switch size.

However, the monorail has such a high intersection area that it is necessary in many cases to completely replace the tracks in switches, which increases their complexity.

The vacuum tube special case

In the monorail tradition, another very radical system has been proposed, where vehicles run within an evacuated tube where the vaccum is thin enough to allow very high speeds (who could be hypersonic).

We wil not dwell here on the technological solutions, but will simply apply the same reasoning we just did to monorails and birails above.

This reasoning, on figure 21 below, will naturally bear no comments, except that the track cross section is superior to the vehicle cross-section.

Figure 21 – Vehicle in an evacuated tube


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