Jim,

BIJ - Billet de Jeunesse International. Sometimes itīs called by different names in the various countries (e.g. Twenticket in Germany), but I think BIJ is the universally-recognized name.

Itīs a discounted youth rail ticket for those who are not yet 26. You can travel for up to two months in one direction, but (at least if bought in Germany - not sure if itīs also true for other countries) you canīt spend more than four days in the country in which you start your journey. You can have as many stops as you want along the way, but since the price is based on distance travelled, itīs cheaper to go in a roughly linear path.
The ticket gives you a discount of 20-50% depending on what countries you travel through. In some countries (like Italy), the base railfare is so cheap that it actually wonīt cost you much to zigzag in a non-linear path. Keep in mind that railfare varies significantly by country (e.g. Germany and Switzerland would not be good places to zigzag ).
One way to make it work for you is to fly into London, get a one-way ticket to southern Europe on a cheapo airline, then go back up with the BIJ (or vice versa).
Yonatan