Object disappearing and origin points I highlight an object in Object Mode but when I click TAB to go into Edit mode, the selected object always disappears. However when nothing is selected, the middle tracks disappears after TAB. What have I done wrong?

I created two railway tracks with one Mirroring the other.
However the origin point is not in the middle of the two rails. Could you please explain:
- why this is the case, and how it ended up there? 
- where the origin point ought to be and how can it be corrected?

I am assuming the origin points of all the objects in the scene are out of whack with each other? Is there anyway to clean up the whole mess?

I am a beginner and I am extremely confused with origin points - I have played around with "origin to geometry" and other "fixes" but I always end up with objects getting displaced or creating more mess. Or when I switch betw object and edit modes, I get objects moving or disappearing. Any advice to a noob would be much appreciated.

Please find attached Blend file.
If you want to display or edit your mesh the way you see in Object Mode but after using modifiers, be sure to select those options (green one for displaying, red for editing) for all modifiers:


What's happening is that the modifiers are not set to display in Edit mode, so when you press Tab you'll get the "original" mesh, without any modifications (in your .blend, for example, you can still see a small piece of your track  in Edit mode).
Also, when you press Tab with nothing selected, Blender will enter Edit mode for the active object (the one with yellowish origin), even if it's not selected. So, probably your track is the active object when you deselect everything.