I bought this book from a library sale in the 90's but the cover art that I can remember would be 80's/70's. I've lost the book but have been thinking about the story for years.
It featured a protagonist who lived in the rear portion of a massive linear city structure that was hundreds of stories high, miles wide and hundreds of mile long. The city 'crawled' across its environs by being deconstructed at the back and the materials and people 'pilgrimmaged' to the front. The story follows the protagonist and his family's final days in the rear portion before they are to undertake the very long move to the front.
The cover was yellow, and I think had a 'Conan'-style depiction of the protagonists in a sewer/tunnel? Any info would be appreciated, Thanks!
Edit: Further details;
The protagonist is a teenage boy who lives in one of the top 'stories' of this structure. There is a massive greenspace/park which takes the form of an atrium that his floor and several others are adjacent to, with a description of massive lights at the ceiling of the atrium.
The pilgrimmage or relocation journey to the front of the city-snake is done using a train-like vehicle at ground-level but the city itself is immobile -- it cannot move on its own except by being built in one direction and un-built in the opposite.
The protag's family, over generations, has changed in class designation based on their position within the city, first at the front as immigrants and some other level by the time the city's back end has caught up to them - it's falling apart and disheveled.
At some point the engineers come to deconstruct his home and there's descriptions of looking out of the disassembled walls over the plains at other cities (?) moving 'like worms on the plain' (definitely not verbatim)