I would suggest this:
/summon Ghast ~ ~1 ~ {CustomName:"Bob the Friendly Ghast",CustomNameVisible:1,ExplosionPower:0,Attributes:[{Name:generic.attackDamage,Base:0}],Invulnerable:1,PersistenceRequired:1,Silent:1,Riding:{id:"MinecartRideable",Inv ulnerable:1}}
But that'd be cheating
