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.
I used to have a good collection of serpent screamer Rollercoater K'nex, I build some layouts, but sold all mine to purchase the Faller fairground model kits.
There are some great K'nex coaster video builds on Youtube