it's been said before, but basically LW ran two trains - a blue, and a red train. The switch track is behind the station between the brake run and the load platform. Next time you ride, look left and back as you roll through to the station (or check it out from the queue) and you'll see a separate section of track leading into the maintenance bay.
As LW got on in years, and train bits became harder to get \ expensive, the park 'cobbled together' a single train from the red and blue trains, which we came to call the 'purple' train (being both red and blue). In it's later years, LW ran only one train (purple).
When the new 'train' was installed as part of the Arkham re-theme, the existing hardware - bogeys etc were kept, and only the seats were changed. So the rolling stock on-track is still the same 'purple' stock previously part of LW. For them to go back to two-train operation, they would have to invest in an entirely new train - rolling stock and all, which would cost more than it did to install the Kumbak seats.