Callback: Wagon length
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GRFv8 This callback is only called for GRF version 7 and below. For GRF version 8 and above use callback 36.
This callback is used instead of train property 21 resp. road vehicle property 23 whenever the vehicle leaves a depot or is displayed inside a depot. If the callback fails, the value of the property is used instead.
The callback result may change only when whole vehicle chain is inside a depot.