Or you could set up a treadmill, and you get a kilometre of free train transport for every kilometre you walk on the treadmill. Call it performance art.

Interesting idea, but I don't think many metro systems are set up to easily sell 'a kilometer' of transport?

Just make it work out on average.

I think you phrased this a little too glibly, but it's a very good point. You can tune two systems to have roughly the "exchange rate" that you'd like to see (unless each is highly sensitive to the other), even if there's not a direct point-for-points correspondence, but it's not necessarily easy to think of that.

Do you mean it's possible without changing the existing ticket systems, or that it's possible to adapt the ticket systems so it could work? For instance, the systems I've used sell tickets either by zone (unlimited travel within a region for x time) or by train station, where the difference between stations varies widely but the cost between stations doesn't.

I think you can mostly make it work for any existing ticketing system.

By `mostly work', I mean `good enough for an art project'.