cannot see camera rig rail mesh in unreal 5

it should work out of the box. Check if you have the corresponding files

Rail:
“/Engine/EditorMeshes/Camera/SM_RailRig_Track.SM_RailRig_Track”
“/Engine/EditorMeshes/Camera/SM_RailRig_Mount.SM_RailRig_Mount”

Crane:
“/Engine/EditorMeshes/Camera/SM_CraneRig_Base.SM_CraneRig_Base”
“/Engine/EditorMeshes/Camera/SM_CraneRig_Arm.SM_CraneRig_Arm”
“/Engine/EditorMeshes/Camera/SM_CraneRig_Mount.SM_CraneRig_Mount”

Check if you have show => Static Meshes unchecked

If you are missing the above files do a verify check of your engine install.