What's the most important (railway) thing you have lost?

[Cooper]

I've 'lost' several items of stock, but they always turn up in an unexpected box where I'd previously put them.    My Oakwood Press book on the Harton Tramway has been AWOL for years now though, so I guess I lent it out and it's gone for good. Now i properly miss that.... 

[terrysoham]

A railway station at Soham 

[Oldman]

A season ticket 24hrs after purchase - turned up 3 days later