That'll teach me to be flippant about things I know next to nothing about in future!
My whole thought process was based on the idea that the West Stand of Lansdowne Road in Dublin was built over the tracks in 1908 but then, I imagine the old legislation and regulation side of things wasn't quite the same back then! It's not as wide a set of tracks either although space would always have been tight in that area.
The problem with Old Trafford is that nothing else can really be done now until the South side is dealt with. It's fine at the moment while we're still head and shoulder above any side in the country, but at some stage surely even the Glazers are going to want to see progression. If Madrid, Barca and Bayern are all sitting there with enormous, shiny stadia to show off, it becomes a status or image symbol as much as anything else.