Preventing men from entering the special carriage for women in Tehran metro

The recent debate on the introduction of women-only carriages for British trains, as put forward by Labour leader Jeremy Corbyn in 2015 revived by a Labour MP in 2017,  briefly directed attention towards the countries in which this option is already available: Iran probably makes the most extensive use of this type of segregation; separating the sexes entirely on buses, but not mainline trains, and providing women-only carriages on its metro system. This is probably the most striking feature of the service to someone in the West, but there are other interesting factors about this particular metro.