@yosh Uh, uhm. I took a PhD course on how to model this and arrive roughly at that number (long story on how I was allowed to do this, even though I only hold a Masters). We did this on server-client systems, but what is a train but a request traveling through the internet.
Essentially, you can take timed petri-nets (a form of state machine with multiple states being active at once - think "trains on routes, in stations") and use a statistical function to model how long it takes for an token (train) in that net to go to the next state (every edge has a different function, runlength). You can then actually reason _locally_ about states at every node using markov chains. ("how many trains are at this station at any time with which probability") and then _back_-calculate that into delay probabilities.
Biggest learning: this isn't linear. It's almost linear, until the point it breaks, where delays grow rapidly and catastrophically over the network.
Which is also why 70% system load is about the moment where you should start thinking about upgrading your servers.
Sorry for the brain dump :).