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.
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