> Stiction etc.? Who knows, none of the trolley system exists yet except in=
 my
> head. I plan to use a sliding barn door track & roller system as the basi=
s of
> the trolley (the type where the door hangs from rollers running in an
> overhead track. I plan to mount the trolley vertically possibly using rol=
lers
> both at the top & bottom. This stuff is quite rugged & should operate for=
 a
> couple of years outdoors in our climate.

I would suggest you talk to guys that do live steam model railways and see =
about some 7.5" or possibly larger track system. Their stuff is also quite =
rugged (the track is typically left out all weathers), and could well be a =
source of rolling items that you could readily adapt to carrying your load.

For distance measurement you could possibly calibrate up an ultrasonic dist=
ance measurement unit quite accurately for setting up just how far you move=
 it, or even a builders tape attached to a known reference point on the tro=
lley measuring to a pointer on the fixed point. It's not clear to me if you=
 need accurate measurements, or if a few millimetres variation each time is=
 part of the point in making the students work through their calculations.

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