Gravitrax compatible magnetic launcher in two versions depending on the size of the magnet.
In my project I used rectangular magnets with dimensions: 15x15x5 mm and 12x10x4 mm.

Launchers are quite powerful. 
I measured that magnet 15x15x5 can shoot the ball to a height of 7.5 cm using on a long hill.
Magnet 12x10x4 can shoot  a height of 6 cm.