I've just retrieved some doohickey for the Railroad, so naturally they want me to join. My next objective is to "Get help to build the Signal Interceptor". To accomplish that I am told to "talk to" the Railroad, Minutemen, or BoS. My inclination is to have the Railroad help me build it. However, the ONLY dialog option I am getting from them is to join the Railroad. I'm guessing that if I approached the Minutemen or the BoS I'd be in the exact same position: Join, or no Signal Interceptor for you! That would be a massive intake of breath through a straw. I would be glad to perform one or more services for any of the groups in exchange, but signing onto any of their agendas is NOT what I want to do. Becoming anyone's True Believer seems ultimately hypocritical and deceptive. Not to mention, if after getting what I need, if I ever decide to resume MY path, the organization would most likely deem me to be a traitor, never to be trusted again, and very probably a candidate for "Shoot On Sight". I'd prefer to be honest and let them know upfront that I would be glad to help them out on occasion, but I'm NOT about to swear an oath of undying fealty to their causes.

So, you want them to get involved and start a war to the finish, but you aren't prepared to join them to do so?

The issue is that he doesn't want to involve the Railroad at all. However, they're the only ones who can decode the Courser Chip, and Desdemona won't allow you to even bring up the subject until you agree to join the Railroad, which the OP doesn't want to do. If there was another way to decode the Courser Chip, he'd probably go with that option instead.