**Downvotes without comment are completely wrong and misleading.** 

This scenario is not at all uncommon.  You will see No Entry/Prohibition signs on many types of trains, including commuter, subway, regional, etc.  Yet, the doors are fully functional and easy to operate.

In these cases, the train operator would really **prefer you not cross** between cars/carriages but the doors must remain **accessible for evacuation** purposes.