I really don't think with disciplines as far apart as cross and track I really don't think you could have one bike that does it all....

The Cross Check is a cross bike - but it could not be converted for track - the drop outs are not horizontal. There are single speed cross bikes, but I really don't know how well they would convert for use on the track.... and then you'd have to race cross with a single speed too.... it is done, but I'm not sure how much I'd suggest it for a beginner.