Re: Locking to railings?
If people don't do this already why would they do it with a bike?
Because people rarely walk around with a chain lock for the purpose of randomly locking someone's gate. Plus that that lock, however cheap it may be, will have cost a few quid.
If, on the other hand, you get to annoy someone by locking the bike you need to park and lock anyway across their gate, then why not? Downside: being traceable via the bike rental database, at least if the rental site has moderately robust credential checking.