Gravity
The force that is causing your toy car to move down the track is gravity. Gravity pulls every object back towards the earth. So when you see your toy car racing down a hill on your track, you're actually watching gravity trying to bring the car back down to earth. Your track just keeps your car from falling straight down. In fact, your track allows your car to fall in a controlled manner and follow the path of the track and make it through all your loops and turns. The higher the ramp height, the more time you allow gravity to try and bring your car back down to earth. This means your toy car will pick up additional velocity if your ramp is higher. In short, longer exposure to gravity through a higher ramp means greater velocity.
Wind Resistance
There is a concept called terminal velocity in physics. This concept tells us that all objects, regardless of size, have a maximum velocity of 9.81 meters per second. Said another way, nothing is capable of falling faster than this. This does not mean your toy car can reach this velocity. Unfortunately for the true toy car speedster, a velocity of 9.81 meters per second can only be reached if no other forces are working against the object to reduce the velocity. A toy car has several. One of the main forces working against your toy car is wind resistance. As your toy car travels it will have to push against the air that is blowing directly against it. Faster velocity and higher ramp heights mean greater resistance. This force, along with others, will eventually keep your toy car from continued acceleration.
Friction
Friction is a force that occurs when two objects rub against each other. It is a competing force that attempts to move an object in the opposite direction than it is moving. To illustrate this point, place your hand on a table and slide it forward. You'll notice that it's harder to move your hand forward while it rests on the table than when moving forward while lifted in the air. This is mostly due to friction trying to pull your hand in the opposite direction. This is exactly what is happening to your toy car's tires as it speeds along the track. Friction between the tires and the ramp will keep the tire from reaching higher velocity. Higher and longer ramps will expose your car to greater amounts of friction. This force, along with wind resistance, will eventually equal the force of gravity and keep your car from gaining additional velocity.