For example: `locations` have many `trains`, `copters`, `trucks`, `ships`. And `trains`, `copters`, `trucks`, `ships` have many `locations`.

moveable_location.rb

#db/migrations/create_moveable_locations.rb

class CreateMoveableLocations < ActiveRecord::Migration
  def change
    create_table :moveable_locations do |t|
      t.references :moveable, polymorphic: true
      t.references :location

      t.timestamps
    end
  end
end

#app/models/moveable_location.rb

class MoveableLocation < ActiveRecord::Base
  belongs_to :moveable, polymorphic: true
  belongs_to :location
end

#app/models/location.rb

class Location < ActiveRecord::Base
  has_many :moveable_locations, dependent: :destroy

  has_many :trains, through: :moveable_locations, source: :moveable, source_type: 'Train'
  has_many :copters, through: :moveable_locations, source: :moveable, source_type: 'Copter'
  has_many :trucks, through: :moveable_locations, source: :moveable, source_type: 'Truck'
  has_many :ships, through: :moveable_locations, source: :moveable, source_type: 'Ship'
end

#app/models/train.rb

class Train < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/copter.rb

class Copter < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/truck.rb

class Truck < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/ship.rb

class Ship < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end