Part of the issue can be down to the fact that few people have an overview of everything that they have to deal with on a project.
I once worked on a project (highways, not rail, but as there's less "departments" to deal with it highlights how much of an issue is likely to be with something which is more complex) where one of the large multi disciplinary consultants was designing the scheme for the contractor.
Problem was that they designed the drainage to pass through the middle of a medium pressure gas main which they had also designed the protection of.
Fortunately, having had a good understanding of the project we were able to highlight this to the client fairly quickly and it was all sorted out before they got to site. Otherwise you can only imagine the costs as the scheme grinds to a halt whilst a work around is sorted out.
The reason that it happened was that drainage and the structure designs were done in different parts of the country and the checking was done in each department rather than by someone who knew the overall scheme.
It's the sort of error which is easy enough to do (which is why I won't say which company did it), however it should also be fairly easy to protect against.
Things like combined drawings (which look a right mess but highlight which bits need to be cross checked for clashes) are helpful as then even someone with zero understanding of what's being shown can ask for clarification as to if (say) that cable can cross that pipe at that location of the two departments who created the drawings showing those elements.
It takes time and therefore costs money, but it's small change compared to the costs which can happen if there's significant delays on site, especially if they then have knock effects for future parts of the project.
I don't know if such issues arose on Crossrail, so the above isn't suggesting that it did. Rather just highlighting how sometimes things can go wrong if there's not the right people asking for the right things.
