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 Mad Monks, you must not have wiped them out and you must have talked to Father Enola before leaving the temple. Assuming the Monks aren't hostile to you on the way through, he will stop you before you can exit the Template to talk about Silo 7 and give you the coordinates needed to send the nuke to them.