Just came across this fantastic creation. Someone has adapted a K'Nex rollercoaster to make it multi-launch.
From the Reddit thread
The trains have magnets underneath them which trigger reed switches in the track, which signal the program in the microcontroller to move on to the next step. I try and upload the code in the morning because it's late here in the UK.
The motors are third party and aren't Knex pieces in a u way shape or form. They take 24v and draw about 2.6A (all the power pack can provide). I can't remember the rpm or torque provided, but its very good. I'll dig up the data sheets in the morning.