The recent announcement by Atlas to release "MTH 18 inch" heavyweight passenger cars and head end cars piqued my interest. The best measurement of car length may be from end of sill to end of sill as any measurement referencing couplers varies by types of couplers and is useless if drawbars are used. As far as I know, MTH heavyweight cars measured 17" from end of sill to end of sill, the same as Jerry Williams' cars. I took these photos this morning
GGD at 18" over the sills
GGD 12-1 with 20 inches over the sills.
K-Line baggage cars at 17 1/2" over the sills
Lionel heavyweight dining car with sounds - 18".
MTH heavyweight baggage car 17". These may be from the same molds as Williams.
Williams heavyweight baggage car 17".
Williams observation car. 17 inches between the sills.