I've been reading this site for years on and off, and I guess now's as good a time as any to actually post something. As of late, I've found myself more and more a fan of local-level infrastructure PODs, largely in relation to transportation (as an urban planning student with a heavy interest in transport planning, that's pretty much a given).
Here's two I can think of just off the top of my head. They both relate to the City of Toronto, because that's where I live, so they'll probably be somewhat incomprehensible to anyone who doesn't know the area.
-What if Toronto's second subway line was built along Queen Street, as planned originally, instead of Bloor Street and Danforth Avenue? The POD is simple (just have the province refuse to budge on changing the location of the line, unlike in real life where they caved to the TTC and agreed to allow the line on Bloor-Danforth), but the effects wide-ranging. Extensions to the suburbs would be much harder, as Queen runs into the lake before even reaching Scarborough, and extensions to the west would be much closer to the lake than they are now, leaving people from further north in Etobicoke with a longer ride to the subway. The Queensway (in Etobicoke, anyway) would be much more important and much more urbanized than it is today, since I see it as the most likely route for western extensions to a Queen Subway. In the east, I could see them sending the line up Greenwood or possibly Donlands (maybe even Pape or Coxwell) to serve East York, then east to Scarborough (possibly through Thorncliffe Park and then east along Eglinton, putting today's Kennedy Station in about the same place as TTL's Kennedy Station). Since the University Subway was a part of the Bloor-Danforth project and it really doesn't make that much sense in a world without Bloor-Danforth, I don't see it as likely to exist, which would put far more strain on the downtown section of the Yonge line by today (seeing as how it's already above capacity in OTL, where there's a parallel line a few blocks away). I could see this leading to the construction of a downtown relief line by the 1990s, probably along Bloor and Danforth, especially when you consider how the Bloor Streetcar (assuming the streetcar abandonment policy doesn't go through ITTL, as it didn't in OTL) would be a rough analogue to today's Queen Streetcar (far over capacity, slow, and unreliable).
For the sake of full disclosure, a TL almost exactly like this was done a long time back by someone on a Toronto transit site I know of. It is pretty much the same as what I said up there, but that's more a function of it just seeming like the most likely option to me than any purposeful attempt at copying him. Note that, since this isn't an alternate history site, the butterfly effect isn't very well represented (at least I think not, haven't read it in a long time), but here's the article if anyone wants to read it:
http://transit.toronto.on.ca/streetcar/4016.shtml
-What if the Spadina Expressway had been constructed as planned, rather than cut off at Eglinton Avenue? Downtown and the areas to the north-west would probably be in worse shape, since the Annex would have been slashed in half and Spadina Avenue turned into a highway through the U of T campus, then widened significantly down to the lake. Also, this would have a significant impact on public transit: the Spadina Subway (opened 1978 in OTL) was originally placed as a higher priority than the North Yonge extensions (opened 1973 and 1974 in OTL), and if the Spadina Expressway had gotten going earlier (which would be IMO the most surefire way to get this alternate timeline going in the first place) I could see the Spadina line being opened to Wilson before Yonge got past Eglinton. In the modern day, traffic coming into Toronto from the north-west would be far more managable, since people could take the highway right into downtown instead of being dumped onto Eglinton at a signalized intersection and forced to take local streets to downtown. Would the construction of Spadina prevent the death of Toronto's freeway plan? I'm not sure, but it seems to me like the type of citizen's movement that ended freeway expansion in the city was only a matter of time in coming, and if it didn't happen with Spadina, it'd probably happen with the next neighbourhood-dividing expressway plan.
EDIT:
Here's another one that just occurred to me. What if Lyn McLeod's Liberals had won the 1995 provincial election, leading to (among other things on a wider scale, obviously) the Eglinton West Subway not being cancelled? Though I'm not sure what the timetable was on construction (shovels were in the ground when it was cancelled), it would have surely been complete from the Spadina Subway to Black Creek Drive by the new millennium. Without the severe capital cuts that the Mike Harris government ushered in, the construction of the Sheppard Subway may have been accelerated as well, with it opening concurrently with or shortly after the Eglinton West line. Assuming both were open by the year 2002 (the year Sheppard opened in OTL), the city's top three rapid transit priorities would have been Spadina North to York University (or more likely Steeles, I'm not sure if the current planned extension to Vaughan Corporate Centre would go through ITTL due to the presence of not one, but two stub subways in Toronto proper), Sheppard to Scarborough Town Centre (possibly with an intermediate extension to Victoria Park Avenue), and completing Eglinton West (not sure of the staging, but eventually to the airport). In OTL, the subway hasn't expanded at all since 2002 due to a perpetual lack of capital funds, but with the politics of the whole thing being different, it's perhaps possible that completing (or at least extending) Eglinton West would become the top priority for the city. In any case, the Black Creek and Eglinton intersection, by 2010, would definitely be significantly more built up, as the plan was to construct a new downtown for York at that location, served by the subway. Amalgamation could throw a wrench into that, but there would at least be some type of high-density development taking root there, given how it's the end point for a new subway line and a major transportation node as a result.