INdeed, in this senerio it the Union will not even have supremancy over North America, this leads to all kind of interesting possiblities in areas that OTL were dominated or even effected by American power.
Hell, Canada's power relatively increases. Will the USA enter WWI? Will the CSA? What about any Cold War?
And it's these easier & more interesting (as well as more palatable!) questions that lead to slavery getting shoved aside.
So let me shove them aside

& say first, it's likely in such an ATL, Canada would end up with Alaska & much of Oregon, with Britain much less sensitive to (lesser) U.S. power. (This also probably means the CNR is built on the more northerly route, thru OTL Jasper; likely it still terminates in Vancouver, which is nearer major markets in the south, & icefree year round. Which suggests the CNR might run on the southern route anyhow...

) Britain is likely to end up with Cuba & the P.I. (perhaps France does) before WW1. During WW1, Japan is liable to be more important to Britain than OTL, & postwar absolutely. (It means the Anglo-Japanese treaty survives, so nothing like OTL's PacWar happens. May also mean a Sino-Japanese treaty against the Sovs.) Possible Japan gets P.I. after WW1, along with the Carolines. TTL's WW1 is liable to go longer & be a draw, with a treaty in 1919 after mutual exhaustion. This means
low probability of WW2. Also IMO means less chance of Britain falling from supremacy in the '20s, & of there being mad inflation in Weimar Germany or a Great Depression. Certainly no Nazis, & Hitler is a house painter or something (or migrates to CSA & becomes a
3d-rate SF writer...

) Can I suggest Jerry Siegel & Joe Schuster create Superman in Toronto, instead?

Or that Jack Kirby, Joe Simon, & C. C. Beck come north, too? (That's a bit farther-fetched, I acknowledge...

)
That said, I have wondered if slavery might not die off as a practise
without secession, & under what conditions.
There is nothing limiting the states as individual or in league or even private ventures from putting money into infrastructure.
A bit OT: doesn't that require involvement from either city or state? That is, if Tony Stark wants to go on a bridge-bulding binge between fights with the Mandarin,

doesn't he have to get permissions from the cities he wants to put them in?