The railroad ending in Fallout 4 is primarily based and focused on sympathy towards Synths. This will make you cross all those limits to ensure the safety and well-being of the Synths. If you choose to be with the Railroad Faction, you will have to go against the Brotherhood of Steel and the Institute.
Fallout 4 Railroad Ending
The Railroad Faction will have you infiltrate the Prydwen airship with the help of Tinker Tom, and plant nitro charges to destroy the ship’s hydrogen gasbags. All these operations will weaken the Brotherhood of Steel. Once the blast of the Prydwen is clear, the charges will detonate, and sending it burning to the ground.
Next, you got to do is destroy the Institute, the steps to destroy the Institute are almost the same for all the Factions. In the Railroad Ending scenario, you got to stay undetected in the Institute as long as a spy from Railroad accesses the inners affairs of the Institute. The spy then helps you plant the explosive charges on the nuclear reactor, which you can detonate the charge and nuke Institute from a distance.
Ultimately, the Railroad ending in Fallout 4 is saving Synths from living a life of slavery. After leaving the Institute building, you will come across a boy, who mistakes you as his parent, you can save this Synth boy. This ending is for you if you believe that the Synths deserve equal rights of life like other human beings.