Seems not, hopefully oberhuemer can shine some light this way as to how he achieved it.
Eddi/planetmaker wrote that code originally. Anyway, this:
8 is maximum and bigger boxes have to done by articulated parts
is exactly correct. In fact, all
vehicles in the CETS/Britrains code consist of three articulated parts. The first part is assigned the actual vehicle graphics, and the curvature between the first/second and second/third part is queried to show intermediate sprites when going around a curve. With this approach, you can have semi-smoothly turning vehicles with a length of between 3 and 16 units, as long as you manage to figure out good-looking sprite offsets. Once again, all credits for this go to Eddi/planetmaker.