Don't carry a spare cable, but I have managed to get the derailleur into a more usable gear. One way is to screw in the high limit screw so the der. is prevented from shifting into one of the smallest cogs. Another option is to manually push the der. inward and then wrap and tie the remaining part of the cable (unless it snapped at the der.) around some part of the bike so it can't move. Either way you now have a 2 or 3-speed bike (depending on whether you have a double or triple chain ring) which is usually good enough to get you home