"Levels" container
From TrainzOnline
The Levels container is a top-level config.txt file entry used by Tracksound assets.
Supported Tags
The levels container has a series of numeric tags in strict ascending sequence, starting with zero. These correspond to the "idle n.wav" sound files in the asset. Each takes a decimal number, which is the maximum speed (in metres per second) that the wave file will be played at - beyond that, the next one up will be used.
Example levels container
levels { 0 0.1 1 5.0 2 10.0 3 20.0 }
This will not play a sound below 0.1m/s, play "idle 1.wav" between 0.1m/s and 5.0m/s, play "idle 2.wav" between 5.0m/s and 10.0m/s, play "idle 3.wav" between 10.0m/s, and play "idle 4.wav" above 20.0m/s.