Introduced in TANE as part of the Train Motion system, the Track Condition for track splines sets the current state/condition of the track. The lower the Track Condition value, the rougher the track will be, and the more the train will sway.
By default, all splines are set to 50% track condition. This should give a relatively smooth track up to approximately 40-50MPH. For track speeds above this, you will need to increase the track condition, with 100% providing perfect track.
The Track Condition default for a track spline can be set with the tag following tag in the track asset's config.txt file: track-condition
The Track Condition can be set either through the Track Properties tool, or using the Get/Apply Track Condition tools, both found in the Surveyor Track Tools Tab