There are no slowdown blocks. If you want to slow down your cart on a certain part of the track and then boost it again, build the following rails:
In order for intersections to work correctly, they must be built with straight rails only (like a +). Building them with a curve in the center will disable minecarts from stopping at intersections. T-Intersections with straight rails work as well.
No special blocks are needed to stop the cart at intersections.
Minecarts will stop at intersections. Continue simply by looking in the desired direction and pressing forward.
Minecarts will automatically slow down before curves or slopes to avoid derailing or crashing into upward slope block. Carts speed up again after curve or slope.
Players can ride the minecart faster even without booster blocks.
Increased maximum speed of minecarts.
Powered Rails can boost the minecart more.
Minecarts will push entities on a collision course out of the way to avoid collision.
Stop/start your cart with a left click.