I was thinking that its unlikely someone is going to ride off on that if you left it unlocked, although shorter forum members may be able to ride standing inside the front tri.
Then again there is a fair amount of tubing there if you did want to make a gate...
I reckon I could tailwhip my mountain bike through the front triangle of one of my mates track bikes!! lol
Biggest head tube going around in Sydney!
*Edit* Actually, we have both extreme's here. Another girl I ride with must have the worlds smallest Vivalo track bike. The head tube cane be any longer than 60mm. Its so short she had to cut her quill stem down so it wouldn't touch the front tire!!