Blue Tit Nest

Last Friday afternoon, I was crossing the bridge over the dual carriageway near Farnborough North station, when I saw noticed a blue tit disappear into a gap in the tubular metal hand-rail.  After a few seconds it came out again and flew off.  then another blue tit with some grass in its beak flew in and disappeared into the hand-rail.  I would have thought that this was a rather exposed place for them to build a nest, but not many people walk over that bridge, so they might succeed.