  The train model which is a variant of the Burridge-Knopoff earthquake model
is investigated for a velocity-strengthening friction law. It shows
self-organized criticality with complex scaling exponents. That is, the
probability density function of the avalanche strength is a power law times a
log-periodic function. Exact results (scaling exponent: $3/2+2\pi i/\ln 4$) are
found for a nonlocal cellular automaton which approximates the overdamped train
model. Further the influence of random static friction is discussed.
