You should be fine to train once the bone has healed around the nail. Whoever put it in should be following you with serial x-rays to determine that the fracture is healing. As long as eveything was put in correctly, you should not be any more susceptible to a fracture than you would be otherwise.
Now, in the event that you are very likely to refracture the leg, i.e. you will continue to ride motorcycles for a long time or you are some wickedly high level kickboxer or striker, you may want to consider having the nail removed after satisfactory union of the fracture has occured. Bear in mind that removal would probably necessitate a period of limited weight bearing and no training.