"their own personal style"
I think that's the answer. There are so many different ways to create a network, rail layout, scheduling, signal configurations, integration with other forms of transport... it's a bit of a creative/aesthetic process and everyone has their own style. If several people cooperated, you may see each person still working in a separate area rather than all combining together to tackle the same transportation sections. Be interesting to experiment with this, however. I wonder if discussions could easily turn to arguments on how best to tackle high volume areas.