The map mentioned by Wesley is included at every InterRail Global pass - it doesn't matters where you buy it.
It is a very simple one.
If you want a paper map with you, think about the one of Thomas Cook:http://www.raildude.com/en/train-schedules
More expensive and with a lot of details is this one:www.europeanrailwayatlas.com
But I recommend you the first one for your rail trip, the second one is too detailed and more for "rail freaks" than for a normal traveler.
Of course the online maps at bueker.net are the most updated ones.