> check the sound/vibration level

You might be able to detect the clicketty-clack pattern as the wheels
pass over the joints. This would help you separate the train from the
background noise

About 1/2 mile from my place is a bridge, which makes a very different,
and louder, sound to that of the train on the embankment. At night I can
hear through the air a long heavy goods train for quite some time as it
heads into town. If there's a landmark feature like that it might help you

Could I suggest you place the microphone or detector at the 2km mark
and RF the event ? You could use a piezo transducer (under a sleeper ?)
to power it, or top up a small battery

Joe
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