  Ever faster computers are enabling us to extend our standard land use
transportation interaction (LUTI) models to systems of cities within which
individual cities compete for resources within the wider environment in which
they interact.As we scale up in this way, we are able to simulate and measure
the impacts of large-scale infrastructures at different spatial levels.Here we
build a platform, which is essentially a digital twin, for over 8000 urban
places in Great Britain where we can rapidly model all flows between these
locations using multi-modal spatial interaction models.We first present the
structure of the model and then apply it to population, employment and trip
flow data for three modes of travel (road, bus and rail) between small spatial
units defining the three countries, England, Scotland and Wales.We then tune
and train the model to reproduce a baseline, and follow this with a
demonstration of the web-based interface used to run and interact with the
model and its predictions.Once we have developed the platform, we are able to
explore variants of the twin, partitioning the country in different ways,
showing how different forms of spatial representation change the performance of
the model.We are developing the model at a much finer scale making comparisons
of performance while adding an active travel layer that elaborates the twin.We
finally illustrate how the model can be used to measure the impacts of new
scenarios for rail, simulating the Integrated Rail Plan and the High Speed 2
proposal
