Enter the Friendly Fire shop in Somerset (the Apartments district) while making sure you have lots of grenades. Throw enough grenades so that one of the doors blows off. The other door will not. Walk out of the store so that you are about one quarter across the road (in between the sidewalk in front of you and the point where the car flow switches directions). Look in the store. You should only be able to see people, cars, and the train. Throw a grenade inside and sprint in. Once you are inside, everything will return to normal. Afterward, you will feel the grenade explode under you. Note: If you do not blow the door off you will not be able to do this because it auto closes.