I just pulled a bunch of my T & Ts out of the closet to check if a scale is listed on the undercarrage or box. The K-line HEAVY HAULERs are marked 1/49 and are larger than either the Lionel or Corgi which were not marked. These HEAVY HAULERs have two different cab styles.
All my Lionel made Tractors have the same cab style. But the Corgi have different cab styles. The Ertl are about the same size as the Lionel & Corgi.
If you have the trucks in hand you can compare the door sizes etc to see if they are compatable. If you are buying on-line I do not know ... GUESS? Like has been said many times in other posts about die cast cars and trucks, SCALE even if listed can vary all over the place. Maybe if you were thinking about a specific item make/number you could post them to see if other forum members who have it could give you the measurements.
Paul Goodness
