Optimizing selection of track sound type

The optimization can be performed in three steps:
1) avoid reselecting track sound type if train stopped
2) directly selecting track sound from .tdb file if there are suitable entries for the track node where the train is
3) assigning at game start or at .ws file load the initial and final track sound for every track node.

Discussion: http://www.elvastower.com/forums/index.php?/topic/28171-optimizing-selection-of-track-sound-type/