The Rustwater Marshes are an area of marshland bordering the Sea of Khazak, considered to be unsafe for Traction Cities since the ground is too soft and muddy to support their weight; cities would get bogged down. They are called the Rustwater Marshes because the water is supposedly red from the rust of towns that became trapped in them. Some very small towns hide here, where the larger ones cannot pursue them. Tunbridge Wheels rebuilt itself to handle the marshland and even cross open water, so that it could hunt down these hiding towns. In the war, the traktionstadts broke through the defences and Dortmund and Darmstadt were bogged down here.
Sources
The Rustwater Marshes appear in Mortal Engines. They also briefly appear in infernal devices when shrike is re-resurrected.