Specifically it seems to me that what makes it tricky is firstly the cramped space and secondly that victoria has three platforms rather than two. The result is that the line from the "original city crossing" doesn't line up with the line into victoria.It's a fairly tricky geometry to tie in with the tight curves into the platforms and the divergence of the 2CC and the original route,
The result is rather than a straight crossing like you would get at a normal two track junction you have two curves sitting on top of each other. The line from the original city crossing doing an S curve to meet the line into victoria and the line from victoria curving off towards the second city crossing.