If you're trying to change the number of exterior cars to 10 just like the normal R-40 Slant, you need to change two things:
1) The train.dat file, where you change the number of motor cars (second number under #CAR) to 10
2) The extensions.cfg file, where you keep adding exteriors until the number reaches 9. You only went up to 6, now add three more.
After you did all that, you should get the 10-car R-40 Slant.