Curtains sliding off rail - what's missing?

[ToolbeltDandy]

I live in a rented apartment and one of the curtain rails has an issue in that the curtains can just slide right off. I'm guessing that the system was designed so that this doesn't happen. What seems to be missing? I have a couple of pictures (click to enlarge). Thanks.



What is this piece?

 

[kbdiy]

Sorry I can't see your pics (problem my end not yours) but there should always be an end stop to prevent this. Depends how old the rails are as to whether you can still buy one or maybe you just need to be a bit creative.

 

[foxhole]

Should be an end stop, small piece looks like a cover for a join.

 

[mattylad]

The small piece is likely the bit that goes in between the 2 ends to join them together.