I often, especially with later-built cities (or no sooner than Republic, at least), rush the aqueduct. Is that not worth doing, if there are forests nearby? I used to routinely cut forests, but stopped once I realized they are 2-shield squares, so I stopped unless I needed the food more.
A town doesn't actually
need a 'Duct until it's at Pop6 and due to fill its food-box in 2T at current harvest: a newly built Duct will then allow the town to roll over to Pop7 without wasting food.
So provided that you can get 100 shields into the Duct-production box before that 2T-deadline, and depending on how many shields you can harvest at Pop5-6, you might not actually
need to start building a Duct until just before/after the town rolls over to Pop6.
If you miscalculate and you get into a situation where you'll hit Pop7 in 2T but the Duct still needs >1T to finish (or we're talking about a more marginal location*, where the town will grow to Pop6-and-three-quarters long before the Duct completes), and
if there is no other way to get the needed shields into the box on
this interturn, then yes, it's worth cash-rushing it. Otherwise, using Forest-chops and unit-disbands is probably preferable to spending your gold.
*But if it's totally marginal (90% corrupt), I wouldn't even bother buliding a Duct there, just farm it.
Why is that? Is the combination of railroad+mine (or RR+any square) equally or more useful? I haven't noticed, and have been leaving forests (and mining) when I find I have more food coming in than I need. E.g., I built very few Hospitals my last game, and only when necessary (I traded for Sanitation fairly late in the game, relatively speaking). In fact, most of the cities I needed them for were conquered ones with lots of space in-between (I even had to built a few new ones in conquered territory).
RR+mine is at least as productive as forest on any square.
True, as
@justanick already mentioned
On grassland you get 1 more food, on plains you get 1 more production.
Absolutely NOT true! Railroads boost the output from the
terrain-improvement: if irrigated, then food; if mined, then shields.
So they do nothing for Forests, not because these tiles are Forests, but because Forests cannot be mined (or irrigated).
On tundra the output is the same, and forest gives reduced movement to attacking troops so forest can be preferable.
Aesthetically, I'd agree that trees are nicer. But once I start founding them, my Tundra-towns will usually be packed CxC, and I'll be chopping Forests for any needed improvements (e.g. Walls or Harbours) so I'll be down to bare snow everywhere. And once that's the case, it's quicker to dig mines than it is to re-plant the Forest (6T vs 9T).