National Park removes coal from the city in question - not from your entire civ. So you can still build RR but the city can't benefit from coal if you build the park.
my entire civ was that city tho, so i figured i'd really lost it. it didn't show up in the resource window any more, after all

.
i built the park so i could run extra engineers, really. but yeah the health helped too. i was at -4 health before i built it, even having saved forests. that that was an extra -2 from already having built ironworks, but +3 from an aqueduct and hanging gardens. and get this ... i mined my iron up north not in the BFC, but forgot to put a road on it

. so i didn't even benefit from the ironworks for several turns, i'm such a permanoob!
the forest preserves let me put off building globe almost forever. i wouldn't have built it, except for the ability to defy resolutions. i had very few happiness resources as you can see! edit: actually, you can't see, since i didn't show my entire borders. the only happies i had were wine and silk

. i did run 10% culture for a little while, rather than build globe early, since i already had NEpic and Parthenon and didn't want the artist points. i ended up getting only 2 (the 3rd was from music).
my production was ridiculously high with all the settled GPs anyway, so i think it was worth it. i think 6 of the engineers got settled (1 did half of space elevator), and at least 10 GSs. so i had a ton of hammers coming in even before the tiles got worked, and i didn't miss the coal bonus.
mostly, it really made me giggle to play a settler always peace game

. it was fun! i think
even i could win that variant on settler without always peace, but it would take longer since i'd have to waste time building military, what a hassle. gilgamesh made 2 or 3 demands, sitting bull never did, wang kon made just one. wang even asked for a DP

! he was at friendly just from civics/religion, such a nice guy. no barbs ever came into my borders. one warrior walked by, that was it. they settled a city to the NW but sitting bull captured it eventually. somebody sabotaged my uranium mine after my ship got launched, so i switched my slider to espionage and researched const/demo for the buildings *giggle*. but that was the only spying i ever saw evidence of.