The length of each piece is 224mm instead of the usual 180mm standard length.
There are stud contacts that protrude from plastic ties.
The top piece in this picture is the usual type of studded M-Track 180mm long.
The middle and lower lengths are the longer type, showing the plastic underside mounted within the metal roadbed.
Information from Koll's catalog (1990)
These 3900 series tracks were the first to have stud contacts, replacing the solid center rail that has been used until then. They were introduced in 1953, initially with a copper contact for the center rail and in 1954 the contact was replaced with the wider nickel plated contacts as seen above.