Go to the Mimic girl's house and talk to her to give you the Train's Pass, then Find the Mimic girl's lost Cleffary doll to get the Train's Pass,After you get the Doll and get out of the Fan club house the Champion Steven ask if you have seen a Lv. 35 LATIOS(LATIAS in HG) and tell you that he(she) is Roaming around Kanto,finally you go to the Mimic girl and she will give you the Train's Pass.
After you captured LATIOS(LATIAS in HG) you get a chance to trade Pokemon with the Champion himself,also after the Trade he will be at Pewter City waiting for you to bring the EGNIMA STONE to him, when you bring it to him, he and the Fossil Extractor man will Examine the Stone that is Actually the SOUL DEW that Bring's LATIAS & LATIOS Together at Kanto,Finally after you get out of the Museum you will Encounter a Lv. 40 LATIAS(LATIOS this time in HG),Be careful if you Defeat her(him) it might be the last time you will see her(him) because it will be the last time you will see Champion Steven so Capture her(him) By Any means Nesassary and try to be Lucky you only get 1 Shot at it.