  We suggest a multilevel model, to represent aggregate train-passing events
from the Staffordshire bridge monitoring system. We formulate a combined model
from simple units, representing strain envelopes (of each train passing) for
two types of commuter train. The measurements are treated as a longitudinal
dataset and represented with a (low-rank approximation) hierarchical Gaussian
process. For each unit in the combined model, we encode domain expertise as
boundary condition constraints and work towards a general representation of the
strain response. Looking forward, this should allow for the simulation of train
types that were previously unobserved in the training data. For example, trains
with more passengers or freights with a heavier payload. The strain event
simulations are valuable since they can inform further experiments (including
FEM calibration, fatigue analysis, or design) to test the bridge in
hypothesised scenarios.
