You may have impressed Desdemona and Deacon and gained their trust after the Tradecraft quest, but you still need to win favor with Doctor Carrington, The Railroad's second in command. Speak to him after Tradecraft to trigger this new optional Railroad quest. His first request is that you pick up a dead drop near Bunker Hill. After doing so, enter Bunker Hill and look for someone named Old Man Stockton at the monument's tavern.
Old Man Stockton adheres to strict discretion. Your first reply in the conversation should be the COUNTERSIGN option, and all replies after that should be the SPEAK COVERTLY choice.
After speaking to Stockton, make your way to the rendezvous point in Cambridge. Before you can make contact at the quest marker, you'll need to clear the immediate area of all hostiles. The locations of possible Super Mutants, Ghouls and Raiders might not be obvious, so be sure to look around. You can also use your VATS to spot nearby enemies.
Once the coast is clear, head to the rendezvous point at the church. Wait there until 11 p.m. Old Man Stockton will then appear with a Synth, H2-22. He'll leave him with you for a brief escort mission. An agent named High Rise will appear shortly after your conversation to assist you.
Follow High Rise for about a mile, taking out all the Raiders along the way to the Ticonderoga Safehouse. You shouldn't have to worry about protecting the H2-22. There aren't that many Raiders to worry about. When you arrive at Ticonderoga Safehouse, speak to High Rise, who will put the Synth under his care.
Return to Railroad HQ and speak to Dr. Carrington to complete the mission.