Tokyo to Hiroshima? Not even close. Try 5 to 5-1/2 hours each way, not counting the 20-30 minutes each way to/from Icky. 11 or 12 hours sitting on trains in return for an hour or so "looking around" sounds like torture to me. If you build in an overnight at a cheap business hotel, you could actually see the Peace Park and some other sights and sample the local cuisine specialties.