Make a railway bridge that spans a ravine/large gap that builds itself as you travel over it. It will unbuild itself behind you as you continue traveling. The clock will unclock itself after a set time (IE: The time it takes to cross the gap or slightly more time), meaning the railway building clock has to be reactivated.

Make it so that the system can only be activate by a minecart with a user in it, mobs/etc. will not trigger the system, causing the mobs to fall to their demise at the bottom of the ravine/etc. The bridge has to be two-way.