Why is monorail more expensive than the conventional rail?

The reason monorail in the tunnel (or at grade) is more expensive than the conventional rail in the tunnel (or at grade) is its suspension.  The entire set of the beam, the bogies, the wheels, etc. is more complex and expensive for a monorail.  Monorail requires a lot of stuff around the beam, often made of expensive rare metals, with a lot more maintenance. Monorail beam itself must be built and maintained to high specifications. The two conventional rails can often be adjusted routinely by "smoothing" equipment set on a moving maintenance train; the monorail beam cannot be handled so easily.

So, what's the main reason for the suspension set of the monorail (beam plus bogies) to be more complex and expensive than that of the conventional rail (be that heavy or light rail)? In monorail, the structure and the running rail are the same thing. This structure/rail must be both strong (to support the weight) and precise (to provide a smooth ride), and these requirements are not always mutually compatible. The entire monorail structure needs to be constructed to a high level of precision. This is particularly difficult and costly due to the unevenly distributed dynamic stress in the sections of the beam between the supports. The sections of monorail beam between the supports are much longer than the distance between the cross-ties of the usual track, and thus the monorail beam FLEXES. All in all, it all comes down to continuous medium elasticity and physics.

On the contrary, the entire idea of the conventional rail is in the separation of the structure and the running rails. The high precision is not required when laying the structure, this precision is reached when a much simpler operation of laying the track is conducted.  In the conventional rail, you solve two engineering problems separately: (1) you design the structure the simplest (and cheapest) possible way to support the weight, and (2) you lay rails with lots of support (at every cross-tie) to smoothly guide the vehicle.  When you lump these two problems together, it becomes a substantially more complex task, with a substantially more complex (and costly) solution.

The only time this entire elaborate monorail suspension makes sense is on the elevated structure, where you save on the structure itself.  The beam is more expensive than the rail, the suspension and the bogies are more expensive than those of the conventional rail, but the structure itself is more narrow and thus compensates for the cost of the suspension, that's how it comes to about the same price for the monorail and the light rail.  However, in the tunnel, the cost of the tunnel is pretty much the same, and the high cost of the monorail suspension (beam plus vehicle) is not compensated by anything.

The main point is: The monorail is DESIGNED for the entirely elevated lines. And I think it should be generally obvious that building a solution designed for the elevated lines in the tunnel is kind of using a wrong tool. How in principle can this be cheaper than using the proper tool, designed without any complications of the "elevated" solution?

Last update: October 14, 2006.