I have an old non-Lion Chief Hogwarts set at least 10 years old where the engine easily pulls half a dozen cars. I did notice a variation of the passenger cars when I ordered additional ones about 5 years ago used that turned out to have black tops. There was also a variation in car numbering (found on each side of car) - 99718 with a gray top, 99719 and 99720 with black tops and 99720 with a gray top. Other than the tops all the cars appear to be identical. I did purchase the dementors coach with sound which had the number 99723. I agree that the sound does add to the effect.
When I get a chance I will provide pictures.
John