For anyone interested, here's what they kitbashed for Trolley II (the tiny trolley on the Neighborhood model.)
It looks a lot like an HO scale motorized streetcar/trolley made by Tyco in the 1950s-1980s in a variety of colorways and decos. (Early runs were possibly branded Mantua rather than Tyco.) In its original state it has a single overhead pole and four motorized wheels.
More info on the Tyco trolley:
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HO Collector article (partial)
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List of releases on a Tyco fansite
The kitbashing needed seems to amount to:
* Cutting out the doors and putting in some floors and walls behind them
* Cutting out the window area entirely as well as the signage stripe below it, but leaving the plain stripe below that.
* Putting in a new backing behind the windows to portray the neighborhood trolley's seats on a black background
* Using some plain sticks of styrene or whatever to create the new larger frames on the sides, you want four squares on a side. (In some older photos it looks like they may be just painted lines on the seats background, in later ones they're definitely glued on.)
* Get rid of the pole on top and attach a pair of the yellow roof signs
* Paint!
Paul Lally (who built Trolley II) was kind enough to post a Model Railroader article at the link Muppetman03 shared above. In the artilce Lally says he kitbashed a Bachmann trolley, but I think that he may have just misremembered. None of the Bachmann trolleys I found photos of look like the version used here.
It seems in earlier seasons of Trolley II's use the roof signs were left plain yellow, while in later seasons you can just barely make out in the ancient broadcast resolution that the roof sign has "Neighborhood Trolley" written on it to emulate the bigger model in Mister Rogers' house. Photos of the model neighborhood from modern displays retain the lettering. If you're doing a replica, you can decide whether or not you want to have the writing on it; both versions are accurate.