Don't underestimate the helpfulness of user forums! During the Session 2 of the UVa Summer Enrichment Program, the class and I were impressed by Kodu's rollercoster potential using winding terrain, "paths" and "first person" camera, but disappointed that when you went to change the velocity of your character, it would literally come off the rails and fall off the path, stuck. Of course, the best part of a rollercoaster is your speed variability! So, with class feedback, I posted to the Kodu user forum, and within a week, got responses, from the Microsoft Kodu development team no less.  Their suggestions:

Here are a couple of ideas the team came up with here (we haven't tried them yet, so let us know):

**Use sight lines to detect objects;  once detected increase speed ; when out of site, theoretically the speed should descrease.
**Put “bumpers” around the tracks.  Treat it like a real rollercoaster, and make sure that if Kodu goes faster along the rails, it “bumps” along the wall to follow the track.
**Using items to control the speed is the right way to go.  You can use the “Nearby” modifier to help.  For instance, if you put an apple at the beginning of the ascent you could program:
WHEN See Apple Nearby DO Settings Speed(Slow)
WHEN See Apple Nearby Not DO Settings Speed(Fast)
**To help keep the coaster on the rails you can increase both  the straight line acceleration and the turn rate and acceleration (character settings menu).  With the acceleration values turned up the character should be able to follow the path more exactly.

--Kodu Team

So, I'll work on testing their expertise to see what works.