The bulk update request #15714 is pending approval.
create alias leaning_on_rail -> against_railing
As far as I can see, against railing and leaning on rail mean the same thing. They both have a significant number of posts. I picked against railing because it has a wiki already.
It could be that "leaning on railing" would be a better name. Both "against whatever" and "leaning on whatever" are used for other tags...