[18Uruguay] Update Discard train. Only trains shall end up in the ban…

…kpool

lib/engine/game/g_18_uruguay/game.rb

@@ -264,7 +264,7 @@ def operating_round(round_num)
             Engine::Step::Token,
             Engine::Step::Route,
             Engine::Step::Dividend,
-            Engine::Step::DiscardTrain,
+            G18Uruguay::Step::DiscardTrain,
             Engine::Step::BuyTrain,
           ], round_num: round_num)
         end

lib/engine/game/g_18_uruguay/step/discard_train.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require_relative '../../../step/discard_train'
+
+module Engine
+  module Game
+    module G18Uruguay
+      module Step
+        class DiscardTrain < Engine::Step::DiscardTrain
+          def process_discard_train(action)
+            action.train.remove_variants!
+            super
+          end
+        end
+      end
+    end
+  end
+end