Domain Size Issue

Hey all,
I've been trying to simulate the flow over a train in a tunnel for the last few weeks, and have been confronted with errors every step of the way. It's a very high speed train, so the domain require needs to be massive, but Flow Simulation keeps throwing up road blocks. I've tried modelling internally and externally (by mapping the CD to the edge of the tunnel walls).

Strangely, the external (and slightly fudged) method seems to give more realistic results, but its clear from what few results I can get that I need an absolutely massive domain. Does anyone know what the upperlimit for size is? It seems Solidworks has some issues with creating a part bigger than 500m, and with creating a CD longer than about 600m. Once you try stretching the CD longer than 600m, the fluid volume inside settles on a maximum size and stays there, regardless of how much larger the CD becomes. Any ideas? Rapidly running out of ideas myself, heh.

Thanks in advance, Dan

SolidworksFlow Simulation