Momentum (kgm/s) = mass (kg) x velocity (m/s) yeh?
So if the train's going really slowly, say 10 m/s (same as a sprinter), and weighs what, 10 (metric) tonnes (= 10,000kg), so it's momentum = 10,000 * 10 = 1,000,000 kgm/s.
And a fly weighs what 1g (=0.001kg)?
Under the "conservation of momentum", ignoring air resistance etc, this means that if a fly which had a mass of 1g was going at 1,000,000,000 m/s (2,236,000,000 miles an hour), then it could stop a train which had a mass of 10 tonnes and was going at 10 m/s (22.36mph)
Man I'm bored. (Y)