The below trams pertain only to WDW, not Disneyland.
I wouldn't doubt that the current trams are the same first generation maybe with different body panels, etc. After reviewing several other historical videos in my collection from Nov 1971 through to mid 1980's I have found at least 6 different versions of parking trams. I think that based on tug setup of Disney Tug #2 and Disney Tug #3 could be the same tug chassis with body refreshes and engine refreshes.
PHOTO COMING SOON
Airport Tug #1 - This airport tug was with an open-air driver area.
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Airport Tug #2 - This airport tug had an enclosure for the driver. Notice in this pic it has the light blue tram cars that remain after the Disney Tug #1 come on-line. Tram car length at this time was only three cars instead of the common place seven cars of today. video 1973 ft wilderness resort
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Disney Tug #1 - I think that this was either the first tugs that Disney both through Ford that had the overheating problem, or that Disney re-designed in-house with Bob Gurr. They looked similar to Version 2 and 3 below at least structurally. video 1972 MK/TTC Parking
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Disney Tug #2 - I think that this version was an upgrade/refurb of the Disney tug #1. Theses were in the 1980's with brown and orange markings. video 1987 EPCOT Parking
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Disney Tug #3 - This is the current verison tug with the Disney Transport and purple markings. video 2011 MK/TTC Parking
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Disney Tug #4 - This is the new tugs recently announced that are coming online soon. Colors may be different of course.
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Early Boardwalk Area Tug - These trams looked to be the same style as Disney Tug #1, with some fringe added to the driver roofline. Except that they are still running after Disney Tug #2 version is on-stage. Maybe these were a few remaining tugs from the upgrade to #2? they only needed probably two tugs for this area. These ran for a short time in early 1990's. This was when Dolphin/Swan was built and Yacht/Beach was being built and construction was starting on Boardwalk. Oddly enough this ran directly on the wide paths from EPCOT International Gateway and on the wooden promenade in front of current Boardwalk Resort. video 1992 Boardwalk Promenade area