I'm quite sorry for the confusion, I believe it was the following rule:
Cxom says:
o You must use the pre-placed checkpoints for the start and end of your track part. You must end ON the upper platform, not on the lower level of the checkpoint.
What I meant was
not that you have to build going from the bottom up, but that, as there are 2 levels to a CP, you have to use the upper one.
This is what I meant: