When the setting for protected entities is on, and straight rails, (and potentially the other) entities is on, the biters will still try to break them. When the straight rail is broken, the game crashes with the following error

2758.286 Error MainLoop.cpp:1435: Exception at tick 2884084: The mod Rampant, fixed (2.0.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RampantFixed::on_entity_died (ID 4)
RampantFixed/libs/ChunkUtils.lua:850: attempt to index local 'newEntity' (a nil value)
stack traceback:
RampantFixed/libs/ChunkUtils.lua:850: in function 'makeImmortalEntity'
RampantFixed/control.lua:681: in function <RampantFixed/control.lua:484>
2758.287 Error ServerMultiplayerManager.cpp:84: MultiplayerManager failed: "The mod Rampant, fixed (2.0.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RampantFixed::on_entity_died (ID 4)
RampantFixed/libs/ChunkUtils.lua:850: attempt to index local 'newEntity' (a nil value)
stack traceback:
RampantFixed/libs/ChunkUtils.lua:850: in function 'makeImmortalEntity'
RampantFixed/control.lua:681: in function <RampantFixed/control.lua:484>"

The fact that the biters attack what was built before the buildings were protected is correct. But the fact that when destroyed and then restored as "immortal" an error occurs - this has been fixed