On the subway trains, when the crew knows about a problem with the doors, they put up a barrier. There are other doors available for the passengers to use (unless they are very stubborn and demand to still use that door).
[video=youtube_share;vzFob7pq-Lg]http://youtu.be/vzFob7pq-Lg[/video]
Since we are to be able to use all four doors (two wide and two narrow) of the new streetcars to enter and egress, the operator could use a barrier to block a malfunctioning door of the streetcar. The passengers (in theory) could use the other doors and still keep the streetcar in service until a mechanic can get to it and fix it. Has anyone seen that happen in other jurisdictions of the world?