For anyone who wants a full map of the Capitol Wasteland ==>
http://www.nma-fallout.com/content.php?page=fo3-maps
: )

Yeah, I’ve got all the Behemoths. Here’s a quick guide. I’ll start with the one most people meet first. 1. GNR Plaza (D.C): If you just follow the main quest-line, you will come across this.
2. Capitol building (D.C, the one with lots of flags on it and a domed roof): In the large circular room, in the middle of the building, he will be fighting Talon Company Merc’s. 3. Evergreen Mills (Quite a long walk west from Megaton): Right in the centre of the outside in a massive electric cage. There are a lot of slavers around here. 4: This isn’t in a particular place but I found that if you start walking from NW/W of Megaton you will find Jury Street metro station. When you find the station walk west until you find some upturned train cars guarded by Super Mutants. In one of the cars is shopping cart containing a teddy. Take the teddy and in about 20 seconds the Behemoth will appear behind you. 5. Takoma Park (D.C): The final Behemoth is a bitch to find. Head to Farragut West metro (directly east from Megaton and across the river) and enter. Make your way to the Tenleytown/Friendship exit. When you’ve entered, head south to the Chevvy Chase exit. When outside, do a left U-turn to find the next metro, and then enter. On the local map, to the east, take the Vernon Square exit. When outside, head south to the Vernon square East metro, which is pretty hard to find because you have to go through a lot of alleys. When inside just follow it along and fight your way through a lot of Mirelurk Hunters and eventually find Takoma Park. Head straight up the broken road, fighting through the Muties, then take the left. Head to the end then cut through the house. Head straight ahead, over the ruined fence, and fight the bad guys. Turn left through the gate then go up the ramp on top of the bus. In the distance you should be able to see the Behemoth. But before you go charging in; look on the table to see a button. Press it. Watch the fireworks. Then if necessary, finish off the Behemoth. Achievement unlocked. Hope this helped : )