Level 71
First, take the long thin beam and stick it down the little gap in the top platform. On the platform below, put the cart just to the left of the beam, and stand a little beam on the far left of the same platform. Down below, underneath the end of the conveyor belt, line up the big fat beams like dominoes, and stand a little beam on top of each one so there's enough height to reach the switch.
Alternative solution
Gravity fan Luisa sent in this solution for us:
