If you look
here go to Part 3 [on the left] and then to page 123 of the file that opens - this is a page from "amendment No2 to Landing Table Force B" this appears to be the second tide forces due at Omaha Beach - anyway the page shows the loading diagram for a LST and includes 2 "Aligator" one belonging to 458 Amphibious truck Co and one to 459 Amphib Truck Co.
There are some 400 pages in the whole document - I haven't read it all so dont know if there are any more Aligators mentioned [I presume that "Aligator" does refer to Amtracks] but theres two to start with - obviously not used in the assault phase but as ferry craft with a large number of DUKWs
lots of other interesting detail in this documment
Noel