The late Frank Coffey, Tweetsie's chief mechanical officer, told me many years ago, that he liked to roster the 12 for the first and last months of the season and the #190 for the 3 months in between.
The two main reasons for this were: 1 - during the hot summers, the airflow in 190's cab was better and 2) - the trains are lighter at the beginning and end of the season. This was not because the 12 couldn't handle the larger load. All else being equal he simply wanted to pamper the 12 a little more as she was older and the park's namesake.
Best Regards,
Glenn