ahh..

cv5/a_train.c

@@ -36,9 +36,14 @@ void print_train(struct car* first) {
 
 void cancel_train(struct car* first) {
     if(first!=NULL){
-        first=first->next;
-        free(first->next);
-        cancel_train(first);
+    struct car* temp= first;
+    struct car* next;
+    while(temp!=NULL){
+        next=temp->next;
+        free(temp);
+        temp=next;
+    }
+    first=NULL;
     }
 }