I read this novel when I was young. It involved some sort of societal breakdown (I think) and ended with the passengers of a spaceship marooned on a planet covered by a single abandoned city. Some of the passengers were reptilian species but the main characters were human. It could be from 1950 - 1960.
Sounds more like "Star Rangers"
A ship of the Star Rangers of the collapsing Galactic Empire flees from the ships of one of the fleets forming out of the remains of the empire's fleet. The Star Rangers are still loyal to the Empire and Central Control, but hated and hunted by the troops of the new empires.
The ship leaves the normal flight paths to escape, and lands on an unknown planet.
There are ruins all over the planet.
Among the Star Rangers are humans and at least one alien that could be described as reptilian.
They find a bunch of other refugees in one of the cities, and join up with them.
Eventually they discover that the "unknown" planet is in fact the Earth, and that city they are in is where the ships were launched that founded the Empire.
There are still humans on the Earth, but they are primitive - they rejected technology and went back to nature. They aren't immune to some virus or other disease the refugees from the Empire have - some of the natives die. I think there was also a native disease that also killed some real jerk among the refugees - it's been a long time since I read it, though, so I may have that mixed up with another story.
Andre Norton's Galactic Derelict comes to mind:
There's societal breakdown on the scale of a galactic civilisation.
The protagonists arrive on an alien world covered by a vast, ruined, city in a somewhat stolen space craft.
The main protagonists are human but the main other species they interact with are somewhat reptilian.
It was first published in 1959.
However the reptilian species appears to be native to the city world rather coming with the human protagonists by star ship at the time of their arrival.