Older post-Voldy fic, name has some sort of transportation reference.
It's an older fic, made up of several episodes, I think named after transportation stations, maybe Underground stations? I have a feeling that the word 'turnpike' or "turnstyle' is in one of the titles.
It it set after Harry defeats Voldy, but I'm guessing it was written before the last book came out, as Severus is alive and well. When he defeated V, something went wonky with Harry's magic and now he has too much power. He can't control it easily and is afraid of what he might accidentally do with it, as he's had several outbursts that fall just short of disastrous. He approaches Snape, asking him to become his safeguard. Apparently none of H's friends that he might prefer to help him are strong enough to fill the role. The bond that H & S agree to subjugates Harry's magic to Snape's control. If Snape doesn't approve of the purpose, Harry can't cast. Snape is reluctant to agree, as the bond is permanent, and encourages a relationship, but when he understands the danger Harry presents and wants to avoid, he consents and they work their way into a peaceful and amiable relationship and end up having a large family together. There is a scene where Harry uses some of his excess power to renovate a house and creates a couple of large bunk rooms that later on are used as dorms for their many children and/or grandkids. One of the kids or grandkids when they grow up thinks the bond H & S have is romantic and asks for details or help in creating the same bond with his intended. H & S are horrified, as they see the inequality of their bond as undesirable, not conducive to a healthy relationship and only bearable because it is necessary for their particular situation, and do their best to discourage that idea.