In addition to the roll moving across the tracker bar, I've seen
and heard of a couple of additions that I enjoyed.  Ken Caswell of
Austin, Texas, has a beautifully restored Feurich Welte upright
that on the expression mechanism he installed Plexiglas.  You can
see the [expression regulator] stopper moving in and out and watch
the crescendos and decrescendos in real time.

I've also heard of someone that opened the valve chest on their
Orchestrelle and installed a Plexiglas plate, then cut an opening
in the wall behind it so that one can watch it from the back-side.
Reportedly it is quite an interesting thing to watch.

Don't forget the steam calliopes where you can see the steam from
the pipes, and the fact that, although it's now MIDI-controlled,
Disneyland's Welte orchestrion has a roll "moving" across the tracker
bar for the effect.

Eli Shahar