Is it railroading?

Okay here's a good question, and it falls under the "is it railroading?"... I have an idea for a campaign, where there is a common goal in the end.  For example a medallion has been stolen and broken.  The medallion protects the countryside and keeps demons at bay.  The hero's need to find all the parts of the medallion and have it put back together somehow. So its broken up into 4 pieces, spread thru out a sandbox style map.  All the encounters are balanced similar so they can all happen at anytime.  Along the way there will be side adventures, etc.  However is it railroading, if the end goal is to have the medallion put back together? especially if there are NPC's nudging the hero's  to "this place where there is a possibility of the a section of the medallion there".