>Water would be too messy
>
>I don't know if motorisation in some form would work. For example
>direct drive with a stepper or clutching a flywheel

You might be able to try an "inverted" linear motor. My thinking is a piece
of extruded aluminium bar on the bottom of the sled, and a series of
electromagnets along the track as the "field" coils. Drive each field coil
with a PWM waveform would mean that the low inductance when there is no
aluminium "slug" in the field would minimise the current draw, but the PWM
signal could give the necessary low frequency phase change between adjacent
coils. Each coil may need its own set of high current drivers, but
essentially you would need only 3 PWM drive signals.

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