No one has offered correct plastic models of the Blue Comet passenger cars.
However, the length of the MTH passenger cars it pretty close for most of the Blue Comet cars.
The Blue Comet consist was made up of older cars refurbished for the new train. None were standard heavy weight passenger cars. In fact the diners were the only cars with six wheel trucks and they were steel clad wooden cars.
If you are looking to put together a "pretty close" Blue Comet Consist the MTH two axle wooden beam passenger trucks would be a good choice for most of the cars.