After making your way through the Canyon of Titan, you'll arrive at the Titan Temple. You will be given information either by the Diamondback Militia or the Mad Monks on a place called Silo 7, which is north of the Temple and Damonta.
When you go to Silo 7, there are a couple turrets in your way to the Silo but not much else. When you arrive, there is a computer in the middle which can be used to send the Nuke somewhere else on the rail line.
In order to send the nuke to the Diamondback Militia, you must not have wiped them out and you must have met them by finding their "secret base", which is in the southwest corner of the Canyon. To open it, you can either use the key which you can get from the "Shadowy Figure" who stops you shortly after entering the Canyon, or you can use Computer Science skill. Once you open it, they will demand you drop your weapons and you can then meet with their commander.