Why do some subway systems (e. g. Paris, Montreal, Mexico City) have rubber tires instead of steel wheels?
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Steel on steel has extremely low traction which is a big reason trains take so long to get going or stop, they could have bigger brakes but it would lock the wheels which cause it to slide.
This is desired as it means it takes alot less energy to maintain speed due to less friction and can (given enough distance) allow greater speeds.
For shorter routes like a metro this can become a problem as the trains start and stop in very short distances, one method of boosting accreation and braking is to have rubber wheels as these are more gripy at the expense of efficency.
For Paris, its because this technology was developed by the local operator (RATP) in the fifties. They wanted to reduce noise (important for aerial lines) but also increase capacity. Because rubber tires have more grip than steel wheels, it allows subways to accelerate and decelerate much faster. It also allows more important slopes and reduces vibrations.
However, it consumes more electricity on longer routes, generates heat and comes with a non negligible upfront adaption cost and increased fixed costs. So its not always as competitive as steel wheels (that also improved over time).
Now for other cities? Some were built by the RATP (Mexico & Montreal). Others for the above reasons. There are many different technologies, variations, from systems to systems.
Its cheap.
You just need a flat surface where tires can run on. The suspension is far simpler with rubber tires and also you can use existing truck workshopw to do maintenance on the tires.
Also some standards might not apply because of vehicle size, weight or technology. Which again saves cost.
It doesn’t really necessary provide a smoother ride as the vehicle is a bit less stable at higher speeds and the savings on the suspension show.
So overall.. simplicity, cost, weight savings etc.
If I’m recalling correctly in Montreal’s case it is because rubber tires provide for a smoother / quieter ride and since the Montreal Metro system is entirely underground, the greater wear-and-tear rubber experiences over steel due to winter weather is a non-issue.