AVR-Chat

> I have used the Qprox (now Atmel) chips with touch pads.

You may be able to do this by hitting the rail with a short pulse and
measuring ringing or reflection time.
Depends too on how the rail is mounted.

There's always the theremin approach, output is a frequency.
You can auto-zero on slow changes, and accept a fast drop in the
variable oscillator's frequency as the touch event.

Theremins are surprisingly sensitive, at least when properly tuned.
Mine will detect my breathing at about 1-2 meters reliably.