[paul.fr]

The train is travelling at a constant speed, yet the sound i hear is a stuttery chh chh chh...chh chh chh...chh chh chh...sound. Why is that?

[neilep]

There's a man in the front who does that !....he goes chh chh chh into a microphone and this is sent to all cabins.

So, now you know !

[DoctorBeaver]

Isn't that just the wheels going over joins in the track?

[lyner]

What is the frequency of the cha cha cha?
Continuous welded rail is pretty smooth and I seem to remember more of a ssshhhhh  sound but, if there are points near where you are listening you can get a ch each time a wheel goes over the join. Urban tracks tend to be full of points,