The line exits stage right through the back-scene. I have decided to use a low relief bridge to disguise this exit. The exit is just a hole in the back-scene board and also cut in to the back-scene sheet:

The idea of the low relief bridge is that it will just slot in to the hole to make it look more bridge like. Since I want it to recede in the gap on the underside, it also has to be removable to allow separation of the layout from the fiddle yard. 

Here are a couple of images of the low relief bridge from the front and from the back to show how it slots in:

Construction is simple using thick card and a cut down Peco plate girder bridge side. The buttresses are finished with Scalescenes dark blue brick. 

Here are a couple of images of the mock bridge in place, one front on and one from the layout viewers angle:

More scenic work to go on in the foreground and to cover the right hand edge but I must retain the ability to remove the mock up.